Characterization of functional neuropeptide Y receptors in a human neuroblastoma cell line

Biased agonists at the human Y1 receptor lead to prolonged membrane residency and extended receptor G protein interaction

Functionally selective ligands to address specific cellular responses downstream of G protein-coupled receptors (GPCR) open up new possibilities for therapeutics. We designed and characterized novel subtype- and pathway-selective ligands. Substitution of position Q34 of neuropeptide Y to glycine (G34-NPY) results in unprecedented selectivity over all other YR subtypes. Moreover, this ligand displays a significant bias towards activation of the Gi/o pathway over recruitment of arrestin-3. Notably, no bias is observed for an established Y1R versus Y2R selective ligand carrying a proline at position 34 (F7,P34-NPY). Next, we investigated the spatio-temporal signaling at the Y1R and demonstrated that G protein-biased ligands promote a prolonged localization at the cell membrane, which leads to enhanced G protein signaling, while endosomal receptors do not contribute to cAMP signaling. Thus, spatial components are critical for the signaling of the Y1R that can be modulated by tailored ligands and represent a novel mode for biased pathways.

NPY binds with heme to form a NPY–heme complex: enhancing peroxidase activity in free heme and promoting NPY nitration and inactivation

NPY binding with heme enhances the peroxidase activity of free heme, resulting in the important tyrosine nitration, which will attenuate its bioactivity.

Micellar nanomedicine of human neuropeptide Y

Human neuropeptide Y (NPY) is an important biologics that regulates a multitude of physiological functions and could be amenable to therapeutic manipulations in certain disease states. However, rapid (within minutes) enzymatic degradation and inactivation of NPY precludes its development as a drug. Accordingly, we determined whether self-association of NPY with biocompatible and biodegradable sterically stabilized phospholipid micelles (SSM) improves its stability and bioactivity. We found that in saline NPY spontaneously aggregates; however, in the presence of SSM it self-associates with the micelles as monomers. Three NPY molecules self-associate with 1 SSM at saturation. This process stabilizes the peptide in α-helix conformation, abrogates its degradation by dipeptidyl peptidase-4 and potentiates NPY-induced inhibition of cAMP elaboration in SK-N-MC cells. Collectively, these data indicate that self-association of NPY with SSM stabilizes and protects the peptide in active monomeric conformation, thereby amplifying its bioactivity in vitro. We propose further development of NPY in SSM as a novel, long-acting nanomedicine.

FROM THE CLINICAL EDITOR

Human neuropeptide Y (NPY) regulates a multitude of physiological functions and could be amenable to therapeutic manipulations, which is currently limited by its short half life. Self-association of NPY with spherically stabilized micelles (SSM) protects and stabilizes the peptide in active monomeric conformation, thereby amplifying its bioactivity in vitro, enabling future therapeutic considerations.

Estrogen up-regulates neuropeptide Y Y1 receptor expression in a human breast cancer cell line

Normal breast tissue mainly expresses the neuropeptide Y (NPY) Y2 receptor whereas primary human breast carcinomas express the Y1 receptor (Y1R) subtype. We hypothesized that activation of estrogen signaling systems plays a role in the induction of Y1R. To investigate this possibility, we used estrogen receptor-positive (ER+) human breast carcinoma cell line, MCF-7, and examined the effect of estrogen on Y1R gene expression and its signaling pathways. Saturation binding studies revealed that MCF-7 cells express high-affinity NPY receptor. NPY inhibited forskolin-stimulated adenosine 3'5'-cyclic monophosphate (cAMP) accumulation and mobilized intracellular Ca(2+) in MCF-7 cells. Chronic estrogen treatment enhanced NPY-mediated inhibition of cAMP accumulation by 4-fold and caused a significant increase in Y1R mRNA expression through ERalpha. Similarly, estrogen increased Y1R mRNA expression in T-47D (ER+) but not in MDA-MB231 or MDA-MB468 (ER-) cell lines. Cycloheximide decreased basal Y1R mRNA expression; however, it did not affect its increase by estrogen. Moreover, estrogen treatment of MCF-7 cells did not increase Y1R mRNA stability. The up-regulation of Y1R expression by estrogen is prevented by hydroxyurea but not by nocodazole or IB-MECA (cell cycle inhibitors). Lastly, NPY inhibited estrogen-induced cell proliferation through Y1R. In conclusion, MCF-7 cells express a functional Y1R coupled to both Ca(2+) and cAMP pathways. Estrogen up-regulates Y1R expression through ERalpha. This effect is independent of increased Y1R mRNA stability or new protein synthesis, and likely occurs during S phase completion of the cell cycle. Estrogen plays an important role in the up-regulation of Y1R, which in turn regulates estrogen-induced cell proliferation in breast cancer cells.

Effect of enterostatin on the feeding responses to galanin and NPY

We have investigated the possibility that enterostatin may inhibit the intake of dietary fat by inhibiting either galanin or NPY-induced feeding pathways. Rats, adapted to either high fat (HF) or low fat-high carbohydrate (HC) diets and fitted with third ventricular cannulas were used to study the effects of intracerebroventricular (i.c.v.) enterostatin on i.c.v. NPY and galanin induced feeding responses in satiated rats. An equimolar dose of enterostatin (0.1 nmoles) inhibited, while a tenfold excess of enterostatin abolished the feeding response to galanin in rats adapted to a HF diet. The galanin stimulation of food intake was reduced in rats adapted to the HC diet and this response was less sensitive to inhibition by enterostatin. Enterostatin had no inhibitory effects on NPY-induced feeding in rats adapted to the HC diet and only a small inhibitory effect, at tenfold molar excess, in rats adapted to the HF diet. The ability of enterostatin to bind to galanin or NPY Y-1 receptors was investigated in ligand binding studies. Enterostatin failed to displace 125I-galanin or 125I-PYY from specific binding sites in rat forebrain homogenates or SK-N-MC cells respectively. The data provide support for the hypothesis that enterostatin specifically inhibits a galanin-responsive fat intake system, but indicate that this effect is not modulated by direct interaction with either galanin or NPY-Y1 receptors.

Neuropeptide Y as a partial agonist of the Y1 receptor

In absence of receptor cycling, human/rat neuropeptide Y was found to persistently occupy the guinea pig neuropeptide Y Y1 receptors expressed on the surface of Chinese hamster ovary (CHO) cells (IC50 approximately 8 nM); a lasting occupancy was also evident with active receptor cycling. A similar blockade was obtained with the human neuropeptide Y Y1 receptor (in CHO or SK-N-MC cells). Peptidic antagonists GR238118 (1229U91) and VD-11 blocked the Y1 receptor in the same molarity range. A neuropeptide Y-related Y1 agonist, (Leu31Pro34) human neuropeptide Y, also strongly adhered to the Y1 site. Similar blockade-like occupancy by neuropeptide Y was found with particulates from Y1-expressing CHO cells, and with native neuropeptide Y Y1 receptors of rat synaptosomes. Peptide YY and a related Y1-selective agonist, (Leu31Pro34) human peptide YY, showed a much less stable binding to the neuropeptide Y Y1 receptor with either the intact cells or particulates. The Y1 binding of neuropeptide Y was also less sensitive to chaotropic agents and guanine nucleotides than the binding of peptide YY, indicating a larger stability for association of neuropeptide Y with the receptor. Inhibition of forskolin-stimulated adenylyl cyclase showed a distinctly attenuating agonism for neuropeptide Y, with an activity similar to peptide YY below 1 nM, but considerably lower above 3 nM of the peptides. This activity was largely exerted via pertussis toxin-sensitive G-proteins of Y1-CHO cells. Our findings indicate that signaling by neuropeptide Y via its Y1 receptor could be self-restricting at higher levels of the peptide, in relation to a strong association of the agonist with the Y1 binding site.

From Transcription Profile to Expression: The Signaling Repertoire of the SK-N-MC Neuroepithelioma Cell-Line

SK-N-MC neuroepithelioma cells are routinely cultured and widely used as a model system in biochemical and pharmacological experiments. To clarify the gene expression patterns of SK-N-MC cells with respect to G protein-coupled receptors and signaling network components, we describe in this report the transcription profile of the cell line. Following the traditional pathway from genome to proteome, selected examples are further examined at the level of protein expression and by functional assays. cRNA targets derived from total RNA extracts were hybridized to Affymetrix Human Genome U133A GeneChip arrays, and the data were analyzed and grouped according to functional aspects. Results obtained for neuropeptide Y (NPY) Y1, Y5, and orexin Ox1 receptors were confirmed by RT-PCR. It is surprising that we found the presence of both NPY receptor subtypes and the absence of the orexin receptor at the mRNA level. Receptor-binding experiments confirmed NPY binding of the Y1 receptor in the nanomolar range but gave no evidence for high expression levels of Y5 receptor subtypes on the cell surface. Protein expression was assayed with immunoblots by using antibodies directed against selected Galpha protein subunits. The presence of at least Galphas, Galphai3, and Galphai2 subunits was indicated.

Corticotropin-releasing hormone (CRH) expression and protein kinase A mediated CRH receptor signalling in an immortalized hypothalamic cell line

Corticotropin-releasing hormone (CRH) is a 41 amino acid neuropeptide which plays an important role in the stress response in the hypothalamus. We describe the development of an immortalized hypothalamic cell line which expresses CRH. We hypothesized that this cell line would possess the relevant characteristics of parvocellular CRH-expressing neurones such as glucocorticoid receptor (GR) expression and vasopressin (VP) coexpression. For production of hypothalamic cells, embryonic day 19 rat pup hypothalami were dissected and dissociated into tissue culture dishes. They were immortalized by retrovirus-mediated transfer of the SV40 large T antigen gene at 3 days of culture and then screened for expression of CRH following dilution cloning. One cell line was chosen (IVB) which exhibited CRH-like immunoreactivity (CRH-LI) and expressed CRH, VP and CRH1 receptor RNA via the reverse transcriptase-polymerase chain reaction. In addition, the cell line expressed the neuronal marker, microtubule-associated protein-2. We verified that the CRH-LI from IVB cell lysates coeluted with CRH standard via reversed-phase high-performance liquid chromatography (HPLC). Furthermore, oxidation of the lysate converted its HPLC profile to that identical with oxidized CRH standard. In addition, IVB cells exhibited high affinity binding to CRH. Incubation of IVB cells with CRH lead to increases in cAMP levels and protein kinase A activity in a concentration-dependent manner. Incubation of IVB cells with CRH also resulted in increases in phospho-cyclic-AMP response element binding protein (CREB) immunostaining as detected by immunocytochemical analysis. Finally, CRH treatment of IVB cell lines has been linked to CREB-mediated gene expression as determined via the PathDetect CREB trans-reporting system. The characteristics of IVB cells, such as CRH and VP coexpression, GR expression and a biologically active CRH-R1-mediated signalling pathway, suggest that this neuronal cell line may serve as model of parvocellular CRH neurones.

Expression of neuropeptide Y receptors in human prostate cancer cells

BACKGROUND

Neuroendocrine molecules are now believed to play a significant role in the progression of human prostate cancer (CaP), especially in the androgen-independent stage.

MATERIALS AND METHODS

In the present study, we evaluated the presence and the function of the receptors for neuropeptide Y (NPY) in human CaP cell lines (the androgen-dependent LNCaP, and the androgen-independent DU 145 and PC-3).

RESULTS

The presence of high-affinity binding sites for NPY was shown on PC-3 cells (radioreceptor assay). Reverse transcription-polymerase chain reaction analysis indicated that these sites correspond to the Y1 and Y2 receptor isoforms. A Y1 receptor protein (70 kDa) was also detected in PC-3 cell extracts by Western blot analysis. The activation of these receptors by NPY resulted in a reduction of forskolin-induced cAMP accumulation and an increase of [Ca2+]i. Moreover, a prolonged treatment with NPY induced a dose-related proliferation of PC-3 cells.

CONCLUSIONS

By showing that NPY receptors are expressed in the androgen-independent cell line PC-3 and that their activation results in cell proliferation, the present date suggest that NPY-related mechanisms might be relevant in certain stages of CaP, such as the progression of the disease during the androgen-independent stage.

Predominant role by CaM kinase in NPY Y(1) receptor signaling: involvement of CREB [corrected]

The role of Ca(2+)/cAMP-dependent signal transduction and transcription factor CREB in mediating NPY- Y(1) receptor function was investigated in SK-N-MC cells. The Y(1) receptor agonist, [Leu(31),Pro(34)]-NPY, inhibited forskolin-stimulated cAMP production which was insensitive to thapsigargin or the CaM kinase II inhibitor, KN-93. Although activation of the Y(1) receptor leads to an increase in CREB phosphorylation, [Leu(31),Pro(34)]-NPY inhibited CREB phosphorylation in KN-93-treated cells. SK-N-MC cells were also transfected with PathDetect cis-CRE and trans-CREB/trans-cFos reporter genes to monitor the role of Ca(2+)/cAMP signals, triggered by Y(1) receptor, on reporter gene activity. Treatment of the cis-CRE-luciferase expression vector-transfected cells with [Leu(31),Pro(34)]-NPY increased reporter gene activity by 2 fold through a KN-93 sensitive pathway. In contrast, the peptide inhibited forskolin-stimulated luciferase activity. Consistently, [Leu(31),Pro(34)]-NPY induced trans-CREB mediated luciferase activity through a CaM kinase dependent pathway, and inhibited forskolin-stimulated luciferase gene expression. However, no effect of the peptide was observed on trans-cFos- mediated luciferase activity. These findings suggest that the NPY Y(1) receptor induces the expression of CRE containing target genes through the CaM kinase-CREB pathway, and inhibits CRE containing genes when cellular cAMP levels are elevated.

Interaction of neuropeptide Y and corticotropin-releasing factor signaling pathways in AR-5 amygdalar cells

Corticotropin-releasing factor (CRF) is a 41 amino acid neuropeptide which is involved in the stress response. CRF and neuropeptide Y (NPY) produce reciprocal effects on anxiety in the central nucleus of the amygdala. The molecular mechanisms of possible CRF-NPY interactions in regulating anxiety behavior is not known. In the central nervous system, the action of NPY leads to inhibition of cAMP production while CRF is known to stimulate levels of cAMP in the brain. Consequently, we hypothesized that NPY may antagonize anxiety-like behavior by counter-regulating CRF-stimulated cAMP accumulation and activation of the protein kinase A pathway. We have engineered an immortalized amygdalar cell line (AR-5 cells) which express via RT-PCR, the CRF(2alpha), Y(1) and Y(5) NPY receptor. In addition, in these cells CRF treatment results in significant concentration-dependent increases in cAMP production. Furthermore, incubation of 3 microM CRF with increasing concentrations of NPY was able to significantly inhibit the increases in cAMP compared to that observed with 3 microM CRF treatment alone. These findings suggest that CRF and NPY may counter-regulate each other in amygdalar neurons via reciprocal effects on the protein kinase A pathway.

99mTc-labeled neuropeptide Y analogues as potential tumor imaging agents

The possible use of neuropeptide Y (NPY) as a novel radiopeptide has been investigated. NPY is a 36-amino acid peptide of the pancreatic polypeptide family, which is expressed in the peripheral and central nervous system, and is one of the most abundant neuropeptides in the brain. Its receptors are produced in a number of neuroblastoma and the thereof derived cell lines. As structure-activity relationships of NPY are well-known, we could assume where a radionuclide might be introduced without affecting receptor affinity. We applied the novel [99mTc(OH2)3(CO)3]+ aqua complex and PADA (2-picolylamine-N,N-diacetic acid) as bifunctional chelating agent. The peptides were synthesized by solid-phase peptide synthesis, and PADA was coupled to the side chain of Lys4 of the resin-bound peptide. Upon postlabeling of [K4(PADA)]-NPY, 99mTc(CO)3 did not only bind to the desired PADA, but presumably as well to the His in position 26. Since the replacement of His26 by Ala only slightly decreased binding affinity, [K4(PADA),A26]-NPY was specifically postlabeled, and the 185Re surrogate maintained high binding affinity. Furthermore, the prelabeling approach has been applied for the centrally truncated analogue [Ahx5-24]-NPY, which is highly selective for the Y2 receptor. The resulting Ac-[Ahx5-24,K4(99mTc(CO)3-PADA)]-NPY was produced with a yield of only 16%. Therefore, postlabeling was applied for the short analogue as well, again substituting His26 by Ala. Competitive binding assays using (185)Re as a surrogate for 99mTc showed high binding affinity of Ac-[Ahx5-24,K4(185Re(CO)3-PADA),A26]-NPY. Internalization studies with the corresponding 99mTc-labeled analogue revealed receptor-mediated internalization. Furthermore, biodistribution studies were performed in mice, and stability was tested in human plasma. Our centrally truncated analogue revealed a 6-fold increased stability compared to the natural peptide NPY. We conclude that Ac-[Ahx5-24,K4(99mTc(CO)3-PADA),A26]-NPY has promising characteristics for future applications in nuclear medicine.

Novel peptide conjugates for tumor-specific chemotherapy

One of the major problems in cancer chemotherapy are the severe side effects that limit the dose of the anticancer drugs because of their unselectivity for tumor versus normal cells. In the present work, we show that coupling of anthracyclines to peptides is a promising approach to obtain selectivity. The peptide-drug conjugate was designed to bind to specific receptors expressed on the tumor cells with subsequent internalization of the ligand-receptor complex. Neuropeptide Y (NPY), a 36-amino acid peptide of the pancreatic polypeptide family, was chosen as model peptide because NPY receptors are overexpressed in a number of neuroblastoma tumors and the thereof derived cell lines. Daunorubicin and doxorubicin, two widely used antineoplastic agents in tumor therapy, were covalently linked to NPY via two spacers that differ in stability: an acid-sensitive hydrazone bond at the 13-keto position of daunorubicin and a stable amide bond at the 3'-amino position of daunorubicin and doxorubicin. Receptor binding of these three conjugates ([C(15)]-NPY-Dauno-HYD, [C(15)]-NPY-Dauno-MBS, and [C(15)]-NPY-Doxo-MBS) was determined at the human neuroblastoma cell line SK-N-MC, which selectively expresses the NPY Y(1) receptor subtype, and cytotoxic activity was evaluated using a XTT-based colorimetric cellular cytotoxicity assay. The different conjugates were able to bind to the receptor with affinities ranging from 25 to 51 nM, but only the compound containing the acid-sensitive bond ([C(15)]-NPY-Dauno-HYD) showed cytotoxic activity comparable to the free daunorubicin. This cytotoxicity is Y(1) receptor-mediated as shown in blocking studies with BIBP 3226, because tumor cells that do not express NPY receptors were sensitive to free daunorubicin, but not to the peptide-drug conjugate. The intracellular distribution was investigated by confocal laser scanning microscopy. We found evidence that the active conjugate [C(15)]-NPY-Dauno-HYD releases daunorubicin, which is localized close to the nucleus, whereas the inactive conjugate [C(15)]-NPY-Dauno-MBS is distributed distantly from the nucleus and does not seem to release the drug within the cell.

Effects of BIBP3226 and BIBP3435 on cytosolic calcium in neuropeptide Y Y1 receptor-transfected Chinese hamster ovary cells and wild type CHO-K1 cells

The NPY Y1-receptor selective antagonist BIBP3226 exerts a dual control on the cytosolic free calcium concentration ([Ca2+]i) in NPY Y1 receptor-transfected Chinese Hamster Ovary Cells (CHO-Y1 cells). It is a potent inhibitor of the NPY-evoked increase in [Ca2+]i. This can be ascribed to its antagonistic properties for the NPY Y, receptor since its less active stereoisomer, BIBP3435, is much less potent. However, when its concentration exceeds 1 microM, BIBP3226 produces a large increase in [Ca2+]i on its own. This effect is mimicked by BIBP3435 and it also occurs in wild type CHO-K1 cells. These latter cells do not contain high affinity binding sites for [3H]NPY and [3H]BIBP3226 and, hence, no endogenous NPY Y1 receptors. It is concluded that, at moderately high concentrations, the NPY Y1 receptor antagonist BIBP3226 and its entantiomer BIBP3435 are able to increase the [Ca2+ ]i in CHO cells either by stimulating another receptor or by directly affecting cellular mechanisms that are involved in calcium homeostasis.

Monitoring of the internalization of neuropeptide Y on neuroblastoma cell line SK-N-MC

Neuropeptide Y (NPY) is an important neuromodulator in the central and peripheral nervous system. The peptide acts through different NPY receptor subtypes (Y1-Y5, y6) that belong to the family of G protein-coupled receptors. In general, cellular responses to prolonged exposure to agonists of G protein-coupled receptors are attenuated, often through internalization of the receptors and their bound ligands. In this study, a fluorescent labeled NPY derivative was synthesized and characterized to investigate the internalization of NPY in the human neuroblastoma cell line SK-N-MC. Internalization was proven by binding experiments and subsequent acidic washing as well as by direct visualization by means of confocal laser scanning microscopy. Approximately 20-30% of the fluorescent labeled NPY and a tritium-marked NPY were resistant to acid removal of cell surface-bound ligands indicating internalization. Extracellular fluorescent labeled NPY was found to be distributed heterogeneously in a clustered pattern, which suggests that the ligand-receptor complex is collected in pits and caveolae followed by endocytosis.

Structure-activity relationships of a series of benzothiophene-derived NPY Y1 antagonists: optimization of the C-2 side chain

A series of benzo[b]thiophene-derived NPY-1 receptor antagonists is described. Systematic modification of the C-2 substituent afforded a 1000-fold range in Y1 receptor affinity. Appropriate substitution at the ortho and para positions of the C-2 phenyl ether produced a synergistic effect on Y1 binding affinity, which led to the discovery of the most active ligands, 12t (K(i) = 15 nM), 12u (K(i) = 11 nM), and 12v (K(i) = 13 nM).

Synthesis and evaluation of a series of novel 2-[(4-chlorophenoxy)methyl]benzimidazoles as selective neuropeptide Y Y1 receptor antagonists

A series of novel benzimidazoles (BI) derived from the indole 2 was synthesized and evaluated as selective neuropeptide Y (NPY) Y1 receptor antagonists with the aim of developing antiobesity drugs. In our SAR approach, the (4-chlorophenoxy)methyl group at C-2 was kept constant and a series of BIs substituted with various piperidinylalkyl groups at N-1 was synthesized to identify the optimal spacing and orientation of the piperidine ring nitrogen relative to the benzimidazole. The 3-(3-piperidinyl)propyl in 33 was found to maximize affinity for the Y1 receptor. Because of the critical importance of Arg33 and Arg35 of NPY binding to the Y1 receptor, the incorporation of an additional aminoalkyl functionality to the structure of 33 was explored. Methyl substitution was used to probe where substitution on the aromatic ring was best tolerated. In this fashion, the C-4 was chosen for the substitution of the second aminoalkyl functionality. Synthesis of such compounds with a phenoxy tether using the 4-hydroxybenzimidazole 11 was pursued because of their relative ease of synthesis. Functionalization of the hydroxy group of 45 with a series of piperidinylalkyl groups provided the dibasic benzimidazoles 55-62. Among them, BI 56 demonstrated a Ki of 0.0017 microM, which was 400-fold more potent than 33. To evaluate if there was a stereoselective effect on affinity for these BIs, the four constituent stereoisomers (69-72) of the BI 60 were prepared using the S- and R-isomers of bromide 17. Antagonist activity of these BIs was confirmed by measuring the ability of selected compounds to reverse NPY-induced forskolin-stimulated cyclic AMP. The high selectivity of several BI antagonists for the Y1 versus Y2, Y4, and Y5 receptors was also shown.

[3H]-BIBP3226 and [3H]-NPY binding to intact SK-N-MC cells and CHO cells expressing the human Y1 receptor

We have studied the binding of [3H]-NPY and the newly developed non-peptide Y1 receptor antagonist [3H]-BIBP3226 to intact SK-N-MC cells and CHO-K1 cells transfected with the human NPY Y1 receptor gene i.e. CHO-Y1 cells. Whereas the association and dissociation of the specific [3H]-NPY binding was slow, the binding kinetics of [3H]-BIBP3226 binding was very rapid. Saturation binding of both radioligands reveal the presence of an apparently homogeneous population of high affinity binding sites in both cell lines. The corresponding equilibrium dissociation constants are similar for the two cell lines and are close to those obtained from previous competition binding experiments. The specific binding of both radioligands was completely and with high affinity displaced by BIBP3226 and its inactive (S)-enantiomer BIBP3435 was much less potent. Whilst the NPY Y1 agonists NPY, PYY and [Leu31-Pro34]-NPY completely and potently displaced [3H]-NPY binding, they could only displace 70 to 80% of the [3H]-BIBP3226 binding sites in CHO-Y1 and SK-N-MC cells. A possible explanation can be that only part of the receptors are G-protein coupled. In agreement pertussis toxin was found to reduce high affinity [3H]-NPY binding sites in CHO-Y1 cells whereas [3H]-BIBP3226 binding parameters remained unchanged.

Human neuropeptide YY1 receptors exert unequal control of the extracellular acidification rate in different cell lines

The ability of the human neuropeptide YY1 receptor subtype to increase the extracellular acidification rate in different cell lines was investigated by using the Cytosensor Microphysiometer. In CHO-Y1 cells (Chinese Hamster Ovary cells expressing the cloned human neuropeptide YY1 receptor), neuropeptide Y increased the acidification rate by up to 15% of the basal level with a -Log(EC50) of 7.42. As expected for neuropeptide YY1 receptors, this response was potently inhibited by the neuropeptide YY1-selective non-peptide antagonist BIBP3226 ((R)-N2-(diphenylacetyl)-N-[(4-hydroxy-phenyl)methyl]-D-arginine amide). Its enantiomer BIBP3435 ((S)-N2-(diphenylacetyl)-N-[(4-hydroxy-phenyl)methyl]-D-arginin amide) was less potent. The antagonists themselves did not affect the extracellular acidification rate at concentrations up to 10 microM. In SK-N-MC cells (a neuroblastoma cell line of human origin that expresses the neuropeptide YY1 receptor) no change of the acidification rate could be observed in the presence of neuropeptide Y at concentrations up to 1 microM. For control, the neuropeptide YY1 receptors were also investigated by assessing whole cell radioligand binding and, at the functional level, by assessing their ability to decrease the forskolin-induced accumulation of cAMP. The specific (i.e., neuropeptide Y-displaceable) binding of [3H]neuropeptide Y was to a homogeneous class of high-affinity sites in both SK-N-MC and CHO-Y1 cells. The equilibrium dissociation constants for [3H]neuropeptide Y, the total number of binding sites and the kinetic constants for association and for dissociation were similar. Neuropeptide Y produced a dose-dependent inhibition of forskolin-induced cAMP accumulation in SK-N-MC cells (-log(EC50) = 9.40) but it did not affect cAMP accumulation in CHO-Y1 cells. Non-transfected CHO-K1 cells were used as negative control throughout the study. No binding or response could be observed in these cells. Our data suggest that the signalling mechanisms of neuropeptide YY1 receptors are closely related to the cell type in which they are expressed.

Structure-activity relationships of a series of 1-substituted-4-methylbenzimidazole neuropeptide Y-1 receptor antagonists

The characterization of a novel series of NPY-1 receptor antagonists derived from the 4-methylbenzimidazole 4 is described. Appropriate substitution on the piperidyl nitrogen of 4 led to systematic increases in Y-1 receptor affinity, to approximately 50-fold, and to the discovery of the importance of a second basic substituent.

Binding of enterostatin to the human neuroepithelioma cell line SK-N-MC

SK-N-MC cells were found to possess binding sites for enterostatin, a peptide with central effects on appetite and sympathetic activation of brown adipose tissue during high-fat feeding. Scatchard analyses of the binding indicated one high-affinity binding (Kd = 0.5-1.5 nM) and one low-affinity binding (Kd = 15-30 nM) for 3H-enterostatin (APGPR). 125I-YGGAPGPR showed similar binding parameters as for the low affinity binding of 3H-APGPR. Met-enkephalin and beta3-casomorphin1-5 were found to displace the binding of 3H-APGPR to the SK-N-MC cells. Affinity purification of solubilized cells revealed an APGPR-binding protein estimated to 53 kDa which may represent a distinct enterostatin receptor. Cross-linking of 125I-YGGAPGPR to intact cells labeled one major protein with the same molecular size. There was no binding of enterostatin to four other human neuroblastoma/neuroepithelioma cell lines, named IMR-92, LAN#5, NB-1 #14 and SH5-SY.

Retinoic acid-induced regulation of neuropeptide Y receptor expression and function in the neuroepithelioma line SK-N-MC

Neuropeptide Y (NPY) acts through specific receptors to inhibit adenyl cyclase and may have a role in neuroblastomas and neuroepitheliomas (NE) as a regulator of cell growth and differentiation. The authors have examined the status of NPY receptors in the NE cell line SK-N-MC and the effect of retinoic acid (RA), a known differentiating agent, on their expression and function.

METHODS

Competitive NPY binding studies were performed on normal and RA-treated cells, followed by Scatchard analysis. NPY receptor function in the absence of or following RA treatment was determined by the ability of various concentrations of NPY to attenuate the forskolin-stimulated accumulation of intracellular cAMP. The mitogenic effect of NPY was evaluated by growing normal or RA-treated cell in the presence of various concentrations of NPY.

RESULTS

Scatchard analysis showed a Kd of 2.3 nmol/L and a Bmax of 91,000 receptors per cell for untreated cells. RA treatment decreased receptor expression to 11,700 per cell without a significant change in receptor affinity (3.6 nmol/L). The effect of forskolin was inhibited by NPY in a dose-dependent fashion in both untreated and treated cells indicating functional receptors in both NPY stimulates the growth of normal SK-N-MC cells. NPY stimulated growth was significantly attenuated after RA treatment, possibly as a result of decreased NPY receptor expression.

CONCLUSIONS

Treatment of SK-N-MC cells with RA, a known differentiating agent, leads to decreased expression of functional NPY receptors and a concomitant decrease in the mitogenic effect of NPY. This supports the role for NPY in the pathogenesis of NE.

Substitution of D-Trp32 in NPY destabilizes the binding transition state to the Y1 receptor site in SK-N-MC cell membranes

The retention rate of the spin label 3-isothiocyanto methyl-2,2,5,5-tetramethyl-1-pyrrolidinyl oxyl spin label (proxyl) attached to the porcine N-acetyl-NPY peptide and the porcine N-acetyl-D-Trp32-NPY peptide at Lys4 was investigated using SK-N-MC neuroblastoma cell membranes containing the Y1 receptor. The release rate of the spin labeled peptides was monitored by electron spin resonance and the KD was determined by a direct radiolabeled NPY displacement binding assay. The analyses show that for the porcine [Ac-Tyr1N epsilon 4-proxyl]-NPY, the KD was 8 x 10(-10) M and koff was 2.7 x 10(-4) sec-1 yielding a value for kon of 3.3 x 10(5) sec-1 M-1. The [Ac-Tyr1, N epsilon 4-proxyl,-D-Trp32]-NPY antagonist ligand had a value of KD equal to 1.35 x 10(-7) M and koff was 1.7 x 10(-4) sec-1 leading to a value for kon of 1.2 x 10(3) sec-1 M-1. The difference in the kon rates of two orders of magnitude is interpreted as demonstrating the N-acetyl-N epsilon 4 proxyl-D-Trp32-NPY ligand binding transition state to be of higher energy then for the unmodified NPY amino acid sequence.

Intraoperative detection and resection of occult neuroblastoma: a technique exploiting somatostatin receptor expression

Tumor cell expression of specific high-affinity somatostatin receptors has been associated with a favorable prognosis in children with neuroblastoma. The purpose of this study was (1) to document intraoperatively the in vivo binding of the somatostatin analogue 125I-tyr3-octreotide to high-affinity somatostatin receptors expressed on human neuroblastoma, using a hand-held gamma detector; (2) to determine whether gamma-probe detection of radioligand binding to tumor receptors could identify occult malignancy; and (3) to determine the safety and biodistribution of 125I-tyr3-octreotide in children. Six children with stage III or IV neuroblastoma received an intravenous injection of 125I-tyr3-octreotide and underwent operative exploration using gamma-probe detection of radioligand binding to tumor somatostatin receptors. Tissue that demonstrated in vivo binding of 125I-tyr3-octreotide, or that was suspicious for tumor, was extirpated and analyzed by histopathology, immunohistochemistry, and microautoradiography. The biodistribution of 125I-tyr3-octreotide was recorded intraoperatively over time. Tumor tissue from each child also was assayed in vitro for somatostatin receptor expression by competitive binding studies using 125I-tyr3-octreotide. In vivo binding of 125I-tyr3-octreotide to malignant tissue was documented in the five children with a known tumor burden. Seventeen sites of radioreceptor binding were amenable to resection. Histopathological analysis confirmed neuroblastoma in 15 of these specimens. Four of the 15 proven tumor foci were occult malignancies. Every site of histologically proven neuroblastoma demonstrated in vivo binding of 125I-tyr3-octreotide. Five of seven sites histologically negative for neuroblastoma also were negative for in vivo radioreceptor binding. Microautoradiography confirmed in vivo binding of 125I-tyr3-octreotide to tumor cells. Uptake of 125I-tyr3-octreotide in abdominal organs occurred within 15 minutes of injection, was highest in the liver and gallbladder, and decreased over 24 hours. The conclusions were as follows. (1) 125I-tyr3-octreotide binds, in vivo, to somatostatin receptors on neuroblastoma, with 100% sensitivity and 71% specificity. (2) Occult neuroblastoma is found through gamma-probe detection of radioligand binding to receptors. (2) The biodistribution of 125I-tyr3-octreotide reflects the hepatobiliary clearance of this radionuclide. (4) Radioreceptor-guided surgery may safely provide more complete operative staging and cytoreduction of neuroblastoma.

High-affinity neuropeptide Y receptor antagonists

Neuropeptide Y (NPY) is one of the most abundant peptide transmitters in the mammalian brain. In the periphery it is costored and coreleased with norepinephrine from sympathetic nerve terminals. However, the physiological functions of this peptide remain unclear because of the absence of specific high-affinity receptor antagonists. Three potent NPY receptor antagonists were synthesized and tested for their biological activity in in vitro, ex vivo, and in vivo functional assays. We describe here the effects of these antagonists inhibiting specific radiolabeled NPY binding at Y1 and Y2 receptors and antagonizing the effects of NPY in human erythroleukemia cell intracellular calcium mobilization perfusion pressure in the isolated rat kidney, and mean arterial blood pressure in anesthetized rats.

Structure-activity relationships of neuropeptide Y analogues with respect to Y1 and Y2 receptors

Secondary structure investigations, affinities, and activities of neuropeptide Y analogues with respect to the Y1 and the Y2 receptor are reviewed. The results are discussed with respect to the different prerequisites for affinities to both receptor subtypes. The results from a systematic scanning of the hormone using L-alanine and from a large variety of discontinuous and cyclic analogs suggest that two different conformations of neuropeptide Y are adopted at the Y1 and Y2 receptors. Whereas a C-terminal turn structure is suggested for Y1 receptor affinity, an alpha-helical conformation of the C-terminus is afforded for good binding to the Y2 receptor.

Receptor binding profiles of NPY analogues and fragments in different tissues and cell lines

To investigate receptor selectivity and possible species selectivity of a number of NPY analogues and fragments, receptor binding studies were performed using cell lines and membranes of several species. NPY displays 4-25-fold higher affinity for the Y2 receptor than for the Y1 receptor. The affinity of [Leu31,Pro34]NPY is 7-60-fold higher for the Y1 receptor when compared with the Y2 subtype. Species selectivity within the Y2 receptors is demonstrated by PYY(3-36), NPY(2-36), NPY(22-36), and NPY(26-36). It is shown that NPY(22-36) is species selective for the human Y2 subtype (K1 of 0.3 nM) compared with the rabbit and rat Y2 receptor (K1 of 2 and 10 nM, respectively). PYY(3-36) displays highest affinity for the human and rabbit Y2 subtype (K1 of 0.03 and 0.17 nM). The screening of NPY analogues and fragments revealed that highest affinity for the human Y2 receptor is shown by NPY(2-36) and PYY(3-36). In addition, PYY(3-36) and NPY(2-36) are not only subtype selective, but also species selective.

Characterization of the human dopamine D3 receptor expressed in transfected cell lines

A full-length cDNA clone of the human dopamine D3 receptor was obtained by the polymerase chain reaction (PCR) using reverse-transcribed RNA from human brain as the template. The cDNA was inserted into an expression vector which was then stably transfected into either Chinese hamster ovary (CHO), SK-N-MC human epithelioma or mouse CCL1.3 fibroblast cell lines. Post-transfection, the Bmax for D3 receptor expression was 1.9, 1.1 and 0.4 pmol/mg protein in the CHO-K1, SK-N-MC and CCL1.3 cell lines, respectively. The D3 receptor expressed in CHO-K1 and CCL1.3 cells exhibited similar radioligand binding profiles, especially for the D3-selective compound, 7-hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT). Radioligand-binding competition curves of presumed D3 agonists were shifted to the right by the addition of guanine nucleotides and Na+ to the assay buffer. Presumed D3-receptor agonists had no effect on cAMP accumulation in any of the D3-transfected cell lines although cAMP accumulation was inhibited by dopamine D2 receptor activation in D2-transfected CHO and CCL1.3 cells and by activation of the exogenously expressed neuropeptide Y receptor in SK-N-MC cells. Also, D3 receptor activation neither potentiated ATP-stimulated arachidonic acid release from CHO cells nor stimulated inositol phosphate production in CCL1.3-cells although both of these responses were elicited by D2 agonists in D2-transfected cells. We conclude that the signalling properties of the D3 receptor differ from those of its closest homolog, the D2 receptor.

Identification of neuropeptide Y receptors in cultured astrocytes from neonatal rat brain

Specific binding sites for neuropeptide Y could be demonstrated in primary cultures of astrocytes from neonatal rat brain. Neuropeptide Y binding was saturable, reversible, and temperature dependent as revealed by saturation studies and kinetic experiments. Scatchard analysis of equilibrium binding data indicated a single population of high-affinity binding sites with respective KD and Bmax values of 0.43 nM and 6.9 fmol/2.7 x 10(5) cells. Physiological responses induced by neuropeptide Y could be detected in a distinct subpopulation of cultured astrocytes on the basis of two criteria: 1) electrophysiological responses and 2) single cell measurements of changes in [Ca2+]i. In that fraction of cells responding (20-70%, varying among cultures from different preparations), brief application of neuropeptide Y led to a membrane potential depolarization, lasting several minutes. When the membrane was clamped close to the resting membrane potential using the whole-cell patch-clamp technique, neuropeptide Y induced an inward current with a similar time course as the neuropeptide Y-induced membrane depolarization. As detected by single cell microfluorimetric (fura-2) measurements neuropeptide Y induced an increase of [Ca2+]i which was caused by the entry of extracellular Ca2+. Both the [Ca2+]i increase and the electrophysiological responses were unaffected by pretreatment of the astrocytes with pertussis toxin.

Two different signal transductions of neuropeptide Y1 receptor in SK-N-MC cells

The signal transduction systems of the neuropeptide Y (NPY) Y1 receptor were studied in SK-N-MC human neuroblastoma cells. NPY induced an increase in intracellular calcium ion concentration ([Ca2+]i) and inhibition of forskolin-stimulated cyclic AMP accumulation, which were mediated through Y1 receptors. One-min preincubation of cells with phorbol 12-myristate 13-acetate (PMA) inhibited both signal transductions dose-dependently, but its effect on [Ca2+]i was about 100-fold more potent than that on cyclic AMP. PMA had no effect on [125I]BH-NPY binding in SK-N-MC cells and hardly inhibited the endothelin-1-induced increase in [Ca2+]i. Pertussis toxin also inhibited the NPY-induced [Ca2+]i increase 30-fold more effectively than the NPY-mediated inhibition of cyclic AMP accumulation. These results indicate that Y1 receptors in SK-N-MC cells couple to two signal transduction systems that have different sensitivities to phorbol ester and pertussis toxin treatments.

Expression of functional Y1 receptors for neuropeptide Y in human Ewing's sarcoma cell lines

In the human Ewing's sarcoma cell line WE-68, saturation analysis using 3H-labelled neuropeptide Y ([3H]NPY) as the radioligand disclosed a homogeneous population of binding sites with a dissociation constant (Kd) of 4.5 nM and maximal binding capacity (B(max)) of 712 fmol/mg cell protein. Besides the WE-68 cell line, ten other human Ewing's sarcoma cell lines (FM-62, HS-80, HT-78, HT-M1-78, NT-68, RM-82, RS-63, VH-64, WE-M1-68, WE-M2-68) were also found to display NPY receptors with Kd varying from 3.5 nM to 10.7 nM and B(max) = 247-3744 fmol/mg cell protein. NPY, its natural analogues and the Y1-receptor-specific peptide ligand [Leu31,Pro34]NPY inhibited [3H]NPY binding in the potency order: [Leu31,Pro34]NPY greater than or equal to human NPY greater than or equal to peptide YY (PYY) greater than salmon pancreatic polypeptide (PP) greater than human PP greater than porcine NPY13-36 much greater than NPY22-36. In the Ewing's sarcoma cell lines NPY provoked inhibition of forskolin-stimulated cyclic AMP formation by up to 98%. Pertussis toxin alleviated the cyclic-AMP-inhibitory response to NPY. In isolated Ewing's sarcoma plasma membranes pertussis toxin [32P]ADP-ribosylated a 41-kDa protein. The ability of NPY and analogues to inhibit cyclic AMP accumulation paralleled their potencies in displacing radioligand binding. By contrast, a cell line derived from an atypical form of Ewing's sarcoma did not express specific and functional NPY receptors. These results demonstrate that conventional Ewing's sarcoma cells possess Gi-protein-coupled NPY receptors of the Y1 type, which upon interaction with NPY, PYY, and PP mediate inhibition of cyclic AMP generation.

A novel cyclic analog of neuropeptide Y specific for the Y2 receptor

The low-molecular-mass, cyclic analog of neuropeptide Y, [Ahx5-24, gamma-Glu2-epsilon-Lys30] NPY (YESK-Ahx-RHYINKITRQRY; Ahx, 6-aminohexanoic acid; NPY, neuropeptide Y), was synthesized and investigated for receptor binding, inhibition of forskolin-stimulated cAMP accumulation, inhibition of electrically stimulated rat vas deferens contractions and ability to increase blood pressure. Like the linear peptide [Ahx5-24] NPY (YPSK-Ahx-RHYINLITRQRY), the more rigid, cyclic analog showed good correlation between receptor binding to rabbit kidney membranes and biological activity in the vas deferens assay. Binding of this peptide to a new Y2-receptor-expressing cell line was slightly reduced, compared to the linear peptide [Ahx5-24] NPY, however inhibition of cAMP accumulation was even more efficient. Unlike the linear peptide [Ahx5-24] NPY, the cyclic analog did not induce a blood pressure increase in rats. Reduced binding to Y1 receptor-expressing SK-N-MC cells, as well as the loss of capability of signal transduction, suggest that only Y2-mediated activity is preserved after cyclization. The selectivity of the cyclic compound for Y2 subtypes of NPY receptors with respect to inhibition of cAMP accumulation is more than fortyfold increased, as compared to the linear NPY-(13-36) peptide, which has been used to determine Y2 selectivity so far.

G-protein coupling and signalling of Y1-like neuropeptide Y receptors in SK-N-MC cells

We have studied [125I]neuropeptide Y-binding sites and neuropeptide Y-mediated second messenger responses in human SK-N-MC neuroblastoma cells with special reference to the role of G-proteins. Neuropeptide Y stimulated two second messenger responses in SK-N-MC cells, inhibition of cAMP accumulation and mobilization of Ca2+ from intracellular stores. Both effects were completely abolished by pretreatment with pertussis toxin. Binding of [125I]neuropeptide Y to intact cells or SK-N-MC cell membranes was rapid, reversible, characterized by high affinity and low capacity, and had pharmacological characteristics of a homogeneous population of Y1-like neuropeptide Y receptors. In permeabilized cells, [125I] neuropeptide Y binding was inhibited by GTP gamma S in a concentration-dependent manner. Saturation experiments in the absence and presence of GTP gamma S demonstrated a reduction in the number of high-affinity [125I]neuropeptide Y-binding sites without a decrease in affinity of the remaining sites. Pretreatment of intact cells with pertussis toxin completely abolished the inhibition of [125I]neuropeptide Y binding by GTP gamma S. Moreover, pertussis toxin treatment reduced the number of high-affinity [125I]neuropeptide Y binding sites. We conclude that the agonist ligand [125I]neuropeptide Y identifies functional neuropeptide Y receptors in SK-N-MC cells; however, the number of specific [125I]neuropeptide Y-binding sites may not necessarily reflect the number of neuropeptide Y receptors, because the former is affected by the functional state of cellular G-proteins.

Centrally truncated neuropeptide Y analog acts as an agonist for Y1 receptors on SK-N-MC cells

The similarity of neuropeptide Y (NPY) to pancreatic polypeptide (PP), whose X-ray crystallographic structure is known, has allowed computer-assisted molecular modelling of NPY and predictions of its three-dimensional structure. Utilizing these techniques, Krstenansky et al. (Proc. Natl. Acad. Sci. U.S.A., 86 (1989) 4377-4381) reported that a centrally truncated analog of porcine NPY, [D-Cys7-Aoc8-17-Cys20]pNPY, which was designed to maintain the tertiary structure of the native molecule, bound to sites on membranes from mouse brain with even higher affinity than native NPY. As brain membranes may represent a heterogeneous mixture of receptor subtypes, we decided to characterize the activity of this analog on a defined cell line. SK-N-MC cells are a human epithelioma cell line with high-affinity receptors of the Y1 subtype which are coupled to inhibition of adenylate cyclase. (D-Cys7-Aoc8-17-Cys20]pNPY bound to receptors on SK-N-MC cells, but in contrast to membranes from mouse brain, with a lower affinity than pNPY. Furthermore, [D-Cys7-Aoc8-17-Cys20]pNPY was able to inhibit isoproterenol-stimulated cAMP production in these cells. Therefore, it appears that the central amino acids deleted from this analog are not involved in NPY binding, and biological activity can be maintained by conservation of the tertiary structure of NPY around the binding surface.