Gastrin-releasing Peptide Binding Research Articles

Overexpression of gastrin-releasing peptide receptor upregulates integrin expression in neuroblastoma cells

Gastrin-releasing peptide (GRP) acts as an autocrine growth factor for neuroblastoma cells. Overexpression of the GRP receptor (GRP-R), which increases binding capacity for its ligand GRP, has been shown to stimulate cell growth; however, the exact role of GRP on tumor invasion/metastatic properties is unknown. Integrin is a heterodimer composed of α- and β-subunits; 24 integrin isoforms exist, from different pairings among 18 α- and 8 β-subunits. Integrins bind to components of extracellular matrix (ECM) such as collagen, fibronectin, and laminin to mediate cell adhesion, spread, or migration. The purpose of our study was to investigate the effect of GRP on integrin expression in neuroblastoma cells. Methods: To determine the effects of autocrine growth factor, GRP, on integrin expression, human neuroblastoma cell line, SK-N-SH, was stably transfected with pEGFP (control vector) and pEGFP-GRP-R plasmids. We then performed a metastasis specific gene array (SuperArray) assay. The expression of ECM receptor integrins was analyzed by Western blot analysis. The expression of cell membrane integrin protein levels was determined by fluorescence-activated cell sorter analysis (FACS). Results: Human neuroblastoma cells overexpressing GRP-R, which we had previously shown to increase GRP binding capacity, demonstrated marked changes in their cell morphology; cells appeared flat and more adhesive to culture plates. Gene array showed increased expression of integrins β1, α2, and α3 when compared to control cells. Consistent with these data, Western blot analysis showed increased protein expression of integrins β1, α2, and α3 in GRP-R overexpressing cells. These findings were further confirmed by FACS, where GRP-R overexpression resulted in increased expression of functional cell surface membrane integrins β1, α2, and α3. Conclusions: GRP-R overexpression resulted in enhanced mRNA and protein expression of integrins, suggesting that GRP-induced neuroblastoma cell proliferation may also enhance factors responsible for tumor invasion/metastatic potential. A better understanding of this process may allow us to develop novel therapy targeted at inhibiting GRP-induced neuroblastoma cell proliferation and metastasis.

Gastrin-Releasing Peptide-Induced Down-Regulation of Tumor Suppressor Protein PTEN (Phosphatase and Tensin Homolog Deleted on Chromosome Ten) in Neuroblastomas

To evaluate whether aggressive, undifferentiated neuroblastomas express tumor suppressor protein PTEN (phosphatase and tensin homolog deleted on chromosome ten) and to examine the effects of gastrin-releasing peptide (GRP) on PTEN gene and protein expression. We have previously shown that neuroblastomas secrete GRP, which binds to its cell surface receptor (GRP-R) to stimulate cell growth in an autocrine fashion. However, the effects of GRP on expression of the tumor suppressor gene PTEN have not been elucidated in neuroblastomas. Paraffin-embedded sections from human neuroblastomas were analyzed for PTEN and phospho-Akt protein expression by immunohistochemistry. Human neuroblastoma cell lines (SK-N-SH and SH-SY5Y) were stably transfected with the plasmid pEGFP-GRP-R to establish GRP-R overexpression cell lines, and the effects of GRP on PTEN gene and protein expression were determined. A decrease in the ratio of PTEN to phospho-Akt protein expression was identified in poorly differentiated neuroblastomas. An increase in GRP binding capacity was confirmed in GRP-R overexpressing cells, which demonstrated an accelerated constitutive cell growth rate. PTEN gene and protein expression was significantly decreased in GRP-R overexpressing cells when compared with controls. Our findings demonstrate decreased expression of the tumor suppressor protein PTEN in more aggressive undifferentiated neuroblastomas. An increase in GRP binding capacity, as a result of GRP-R overexpression, down-regulates PTEN expression. These findings suggest that an inhibition of the tumor suppressor gene PTEN may be an important regulatory mechanism involved in GRP-induced cell proliferation in neuroblastomas.

Stimulation of proliferation and migration of a colorectal cancer cell line by amidated and glycine-extended gastrin-releasing peptide via the same receptor

Although amidated forms of gastrin-releasing peptide (GRP) have been identified as autocrine growth factors in small cell lung cancer, their role in the development and progression of colorectal carcinoma is less clear. In addition, the biological activity of non-amidated gastrin-releasing peptide has not been investigated in colorectal carcinoma cells. We therefore investigated the effect of bombesin (a homologue of gastrin-releasing peptide) on proliferation, migration and inositol phosphate production in the human colorectal carcinoma cell line DLD-1, and determined the ability of gastrin-releasing peptide receptor antagonists to inhibit these effects. We also compared the biological activities of amidated and non-amidated GRP in the same assays. Treatment with either bombesin, or amidated or non-amidated GRP resulted in significant increase in proliferation, and in migration in a wound-healing assay. Both the mitogenic and migratory effects of amidated and non-amidated forms were inhibited by the GRP receptor antagonist [ d-Phe 6, Leu-NHet 13, des-Met 14]–bombesin(6–13). The presence of GRP receptor mRNA and GRP binding sites in three colorectal carcinoma cell lines was demonstrated by RT-PCR and by binding of radiolabelled bombesin, respectively. Transfection of DLD-1 cells with a dominant negative phosphatidylinositol 3-kinase did not affect bombesin-stimulated cell proliferation, but inhibited bombesin-stimulated cell migration. Bombesin and GRPgly activated phospholipase C, mitogen-activated protein kinase and focal adhesion kinase. We conclude that both amidated and non-amidated forms of gastrin-releasing peptide accelerate proliferation and migration of DLD-1 human colorectal carcinoma cells via the gastrin-releasing peptide receptor, but that phosphatidylinositol 3-kinase is only involved in the cell migration signalling pathway. Our results suggest a potential role for gastrin-releasing peptide receptor antagonists in the management of colorectal carcinoma.

Is there a role for agonist gastrin-releasing peptide receptor radioligands in tumour imaging?

Gastrin-releasing peptide (GRP) has been shown to be a tumour growth stimulating agent for a number of normal and human cancer cell lines. The tumour growth effect is a direct result of GRP binding to membrane G-protein coupled GRP receptors (GRP-R) on the cell surface. Available data on the role of GRP and GRP-R in human lung, prostate, breast, colorectal and gastric carcinoma are reviewed and it is suggested that radiolabelled agonists are preferable to antagonists for imaging and therapy as they appear to be internalised, yielding a higher target/background ratio. The use of rhenium or indium radiolabels for therapy may provide a new approach to GRP/bombesin expressing tumours.

Modulation of gastrin-releasing peptide (GRP) receptors in insulin secreting cells.

Gastrin-releasing peptide (GRP) receptors are present in pancreatic islets, though their regulation is unknown except for homologous desensitization. The modulation of binding of GRP to mouse pancreatic islets and INS-1 cells was studied. At 60 min (steady-state), total binding of [(125)I-Tyr(15)] GRP was 1.62 per cent of total radioactivity per 50 islets; non-specific binding (presence of 1 mM unlabelled GRP(1-27)) was 0.05 to 0.61 per cent of total radioactivity. A preincubation with 1000 nM cholecystokinin (CCK(8)) or with 1000 nM glucose-dependent insulinotropic peptide (GIP) augmented the number of GRP binding sites but not their affinity. [(125)I-Tyr(15)]GRP binding to INS-1 cells was saturable (90 min) and specific with respect to compounds that are not chemically related to GRP (e.g. calcitonin gene-regulated peptide-CGRP and atrial natriuretic peptide-ANP). Displacement studies showed one binding site with a K(d) of 0.39 nM and a B(max) of 13.2 fmoles mg(-1) protein. When the cells were pretreated for 24 h with 10 nM GIP or CCK(8), only GIP but not CCK(8) increased the B(max) of the GRP binding site. The affinity (K(d)) was not changed by either compound. This effect of GIP pretreatment was not affected by downregulating PKC by TPA (phorbol ester; long-term pretreatment). These data indicate that: (1) specific binding sites for GRP are present in mouse pancreatic islets and INS-1 cells; (2) the GRP binding is upregulated by GIP in both islets and INS-1 cells and additionally by CCK(8 ), albeit only in islets; and (3) PKC does not seem to be involved in the up-regulation process. Thus a positive interplay between both the incretins GIP and CCK(8) and the neurotransmitter GRP is obvious.

Bombesin-like Neuropeptides in Nematodes

This paper reports evidence of members of the bombesin-like peptide family in a number of nematodes, including Caenorhabditis, Panagrellus, Dirofilaria, Onchocerca, Brugia, Haemonchus, Ostertagia, Toxocara, and Ascaris. One of these (Ostertagia) secretes the bombesin-like material into its environment. Specific binding of gastrin-releasing peptide to the hypodermis consistent with the presence of receptors was demonstrated. These data suggest that this class of peptides may play an important role in nematode hypodermal physiology.

Bombesin-like peptide receptors in human bronchial epithelial cells.

Northern blot and RNAse protection assays previously failed to detect bombesin-like peptide (BLP) receptors in normal human lung tissue, but by RT/PCR cultured human bronchial epithelial (HBE) cells expressed all three BLP receptor subtypes, predominantly neuromedin B (NMB) receptor. By RT/PCR, we found expression of all three BLP receptor subtypes by human lung tissue and confirmed NMB receptor expression in six out of six HBE samples. However, transformed HBE BEAS B2B cells expressed only gastrin-releasing peptide (GRP) receptors; saturable, high-affinity (Kd = 3.5 nM) specific [125I]GRP binding confirmed functional GRP receptor, with M(r) = 75 kDa and immunologic cross-reactivity with GRP receptor from human small-cell lung carcinoma (SCLC) NCI-H345 cells. Altered regulation of BLP receptors may accompany transformation of normal lung cells to cancer.

The conserved NPXnY motif present in the gastrin-releasing peptide receptor is not a general sequestration sequence.

Exposure of the gastrin-releasing peptide (GRP) receptor to agonists causes a rapid desensitization of the receptor-stimulated mobilization of intracellular calcium. Homologous desensitization occurs by uncoupling the G-proteins from the receptor and by ligand induced internalization. The molecular determinants of desensitization of the GRP receptor are not well known. The importance of tyrosine 324 which is located in a highly conserved NPX2-3 Y motif of the GRP receptor was investigated. Kirsten murine sarcoma virus-transformed rat kidney (KNRK) cells were transfected with expression vectors encoding either the wild type or the mutant (tyrosine 324 to alanine 324) rat GRP receptor. The wild type and mutant GRP receptors were expressed at a high level in the KNRK cells, 2.0 x 10(6) and 0.5 x 10(6) receptors per cell, respectively. The wild type and mutant GRP receptors bound GRP with the same affinity (Kd = 6-7 nM). KNRK cells expressing the wild type or mutant GRP receptor had similar [Ca2+]i, dose response to GRP. KNRK cells expressing the GRP receptor rapidly internalized bound 125I-GRP at 37 degree C. Internalization was inhibited at 4 degrees C and by 0.45 m sucrose. The internalization of bound 125I-GRP by the mutant GRP receptor was identical to the wild type receptor. Fluorescent microscopy was used to directly observe the GRP receptor expressed on the surface of the KNRK cells and to visualize its ligand induced internalization. There was no difference in the pattern of internalization between the wild type and mutant GRP receptors expressed in KNRK cells. Therefore, the highly conserved tyrosine 324 does not have a role in GRP binding, receptor-G-protein interaction, or initial events of ligand induced receptor internalization. The NPXnY motif is not a general sequestration sequence for seven transmembrane G-protein linked receptors.

Novel bombesin-like peptide binding proteins from lung.

Gastrin-releasing peptide (GRP) and other bombesin-like peptides (BLP) play an important role in lung development, response to injury, and carcinogenesis. However, the mRNAs from previously cloned BLP receptors are not detectable on Northern blots of normal lung. The purpose of this study was to isolate and characterize BLP binding proteins from normal mouse lung. Soluble cytoplasmic and detergent-solubilized membrane fractions were prepared from mouse lung and evaluated for specific 125I-GRP binding. Unexpectedly, not only the solubilized membrane but also the soluble cytoplasmic fractions demonstrated saturable, high-affinity, specific GRP binding activity with Kd = 1.6 nM, Bmax = 135 fmol/mg protein and Kd = 7.5 nM, Bmax = 323 fmol/mg protein, respectively. BLP binding proteins were isolated using GRP14-27 affinity chromatography and analyzed by SDS-PAGE. In each fraction, a major unique band of approximate M(r) = 70 kD was obtained and flanked by two weaker bands of approximate M(r) = 65 and 75 kD. Preincubating samples of the cytoplasmic fraction with various neuropeptides demonstrated specificity in that only incubation with GRP14-27, the bioactive portion of the molecule, blocked affinity purification of these BLP binding proteins. The BLP binding proteins isolated from the cytoplasmic fraction were purified by HPLC, digested with trypsin, and sequenced via Edman degradation. These BLP binding proteins yielded peptides with the sequences IXGIYTDGQNTPXG and RAIMVEXXSEAXXSLLTP, both of which are unique compared with the GenBank/EMBL data base.(ABSTRACT TRUNCATED AT 250 WORDS)

Gastrin-releasing peptide antagonists

Small cell lung carcinoma (SCLC) cells contain gastrin releasing peptide (GRP) receptors. The response of the cells to GRP is rapid growth. We have found a group of peptide derivatives that act as GRP antagonists by blocking the binding of GRP to its receptor thereby inhibiting the growth of cells that are sensitive to the growth promoting activity of GRP.

Identification of a 63-kDa serum protein that binds somatostatin and gastrin-releasing peptide but not bombesin

A 63-kDa serum protein was identified that bound somatostatin and gastrin-releasing peptide (GRP) but not bombesin. The 63-kDa protein was detected by its ability to compete with the receptor for GRP in a receptor binding assay. This interaction could be inhibited by the addition of somatostatin, producing a higher and more accurate calculated affinity for the binding of GRP to its receptor. Somatostatin did not affect the affinity of the receptor for bombesin. Specificity of the 63-kDa protein for analogs of somatostatin, GRP, and bombesin was determined by competition with 125I-GRP and 125I-somatostatin. A tripeptide motif consisting of an aryl-hydrophobic-basic amino-acid structure in residues 15–17 of GRP (Tyr-Pro-Arg) and residues 7–9 of somatostatin (Phe-Trp-Lys) was implicated in binding. This tripeptide binding motif is not present in bombesin. That residues 15–17 of GRP are highly conserved suggests that its interaction with the 63 kDa serum protein may be physiological.

Development of potent gastrin-releasing peptide antagonists having a D-Pro-psi(CH2NH)-Phe-NH2 C terminus.

Gastrin-releasing peptide (GRP) is a 27-amino acid neuroendocrine hormone that may play a role in the pathophysiology of small cell lung carcinoma. GRP and bombesin, a structurally related peptide, stimulate the growth of some cultured cell types. C-terminal GRP peptide analogs were developed that inhibited 6 nM bombesin-induced [3H]thymidine incorporation into quiescent murine Swiss 3T3 cells, which routinely produced a 6-fold stimulation over the basal extent of incorporation. The peptides were also analyzed for their capacity to inhibit the binding of 50 pM 125I-labeled GRP to Swiss 3T3 cells. The combination of two chemical modifications, each antagonistic in itself, led to the creation of antagonists with orders of magnitude greater potency than either modification alone. (i) Antagonist analogs of the form -Leu26-psi(CH2NH)-Xaa27-NH2 [where Xaa is Leu, norleucine (Nle), or Phe; residues numbered after GRP], similar to those introduced by Coy and coworkers [for review, see Jensen, R. T. & Coy, D. H. (1991) Trends Pharmacol. Sci. 12, 13-19], were found to have nanomolar potencies. (ii) We found that an octapeptide C-terminal GRP analog having D-Pro adjacent to the C-terminal amino acid amide was antagonistic, with a potency of 40 nM. By combining both modifications, specific analogs were found with potencies > 1000-fold greater than our lead structure--[(4'-hydroxy)-3-phenylpropanoyl]-Pro-Arg-Gly-Asn-His-Tr p-Ala-Val - Gly-His-Leu-psi(CH2NH)-Nle-NH2--and greater than any antagonist previously reported. The analogs [(4'-hydroxy)-3-phenylpropanoyl]-His-Trp-Ala-Val-D-Ala-His-D-Pro- psi(CH2NH)-Phe-NH2 and 1-naphthoyl-His-Trp-Ala-Val-D-Ala-His-D-Pro-psi(CH2NH)-Phe-NH2 antagonized [3H]thymidine incorporation with IC50 values of approximately 0.3 nM and inhibited the binding of 125I-labeled GRP with IC50 values of approximately 1 pM. These peptides may be of use in the study of the physiology of GRP.

Gastrin-Releasing Peptide

The binding of GRP (gastrin-releasing peptide) to mouse pancreatic islets was studied. Binding of 100 pM 125I-GRP to collagenase-prepared isolated islets at 22 degrees C was one-half maximal after 15 min and maximal at 60 min. At 60 min, total binding was 1.62% of total radioactivity per 50 islets; nonspecific binding (presence of 1 microM unlabeled GRP-1-27) was 0.05-0.61% of total radioactivity. GRP binds specifically to a high-affinity site (Kd1 = 0.81 nM; Bmax1 = 12.8 fmol/50 islets). The specific binding is saturable. Hormones with the intact C-terminus of GRP-1-27, such as N-acetyl-GRP-20-27 and neuromedin C (GRP-18-27), possess the same inhibition curve as GRP-1-27. GRP-1-16, with a cleaved C-terminus, does not inhibit binding of 125I-GRP. However, hormones that virtually are not structurally related to GRP, such as eledoisin, galanin, and VIP (vasoactive intestinal peptide) do not compete for GRP binding. The rank order of GRP analogs such as GRP-1-27, N-acetyl-GRP-20-27, and GRP-1-16 is similar though not identical with respect to inhibition of 125I-GRP binding and insulin secretory potency. We found that 1 and 10 nM GRP-1-27, at a stimulatory glucose concentration, increases the breakdown of phosphatidylinositol to Ins-1,4,5-P3, the biological relevant isomer of Ins-P3; 10 nM GRP-1-27 is effective even at a nonstimulatory glucose concentration in this respect. In a virtually Ca(2+)-free medium, 5 nM GRP-1-27 increases the 45Ca2+ efflux from 45Ca(2+)-prelabeled islets. These data indicate that (a) specific binding sites for GRP are present in mouse pancreatic islets; (b) GRP superimposes the maximal insulinotropic effect of glucose; and (c) Ins-1,4,5-P3 is probably involved as a second messenger in the biological effects of GRP-1-27, which is underlined by the efflux of Ca2+ from intracellular stores but is not a sufficient signal by itself.

Gastrin releasing peptide antagonists with improved potency and stability

Gastrin releasing peptide (GRP) is a 27 amino acid peptide hormone which is homologous to the amphibian peptide bombesin. Two series of novel GRP antagonists were developed by C-terminal modification of N-acetyl-GRP-20-27 amide. Peptide derivatives within each series resist enzymatic degradation in serum and exhibit strong affinity for the GRP receptor. The first series of compounds replaces the Leu26-Met27 region of GRP with an alkyl ether N-acetyl-GRP-20-25-NH-[(S)-1-ethoxy-4-methyl-2-pentane], specifically blocked radiolabeled GRP binding with an IC50 of 6 nM. In the second series of antagonists the oxygen of the ether moiety is replaced with a methylene group, resulting in GRP antagonists which are equipotent to native GRP in receptor binding assays (IC50 = 2 nM) and are also resistant to proteolytic degradation in vitro. All of the C-terminally modified peptides tested blocked GRP-stimulated mitogenesis in Swiss 3T3 mouse fibroblasts. Representative compounds also blocked GRP-induced elevation of [Ca2+]i in human SCLC cells, and inhibited GRP-independent release of gastrin in vivo.

Gastrin-releasing peptide is a transmitter mediating porcine gallbladder contraction

We studied the role of gastrin-releasing peptide (GRP) for porcine gallbladder motility. Immunohistochemistry visualized nerve fibers containing GRP-like immunoreactivity in muscularis. GRP concentration dependently stimulated contractions of muscularis strips (ED50, 2.9 nM). Neuromedin B was less potent (ED50, 0.1 microM), suggesting existence of GRP-preferring receptors. GRP-induced contractions were unaffected by muscarinic antagonism (1 microM atropine), axonal blockade (1 microM tetrodotoxin), cholecystokinin (CCK) receptor antagonism (10 microM MK-329), or substance P desensitization (1 microM), supporting the existence of myogenic GRP receptors. The bombesin (BN) analogue D-Phe6-BN-(6-13)propylamide (PA) stimulated contractions (ED50, 3.3 nM) with low efficacy (29% of that of GRP). D-Phe6-BN-(6-13)PA (1 microM) shifted GRP concentration-response curves one log to the right. D-Phe6-BN-(6-13)PA interacted specifically with GRP receptors; while abolishing responses to GRP (1 nM), responses to substance P (0.1 microM) and CCK-8 (1 nM) were unchanged. Electrical stimulation (10 Hz, 0.5 ms, 10 V) caused a rapid onset-slow offset, tetrodotoxin-sensitive excitation. Atropine reduced the amplitude to 58% and caused a delayed, slow onset-slow decline response. D-Phe6-BN-(6-13)PA reduced the amplitude to 59% and caused a very rapid onset-rapid decline response. Atropine plus D-Phe6-BN-(6-13)PA abolished responses to nerve stimulation. Nerve stimulation caused significant release of GRP-like immunoreactivity. Thus two neural inputs were defined: a cholinergic rapid onset-rapid offset excitation and a delayed, slow onset-slow offset excitation caused by release and subsequent binding of GRP to GRP-preferring receptors.

Modulation of inositol lipid hydrolysis in human breast cancer cells by two classes of bombesin antagonist

Inositol lipid hydrolysis was monitored in the human breast cancer cell line MCF-7 in response to various bombesin (BN) and substance P (SP) analogues. Both members of the BN family of peptides, i.e. BN and gastrin-releasing peptide (GRP), stimulated a dose-related increase in total inositol phosphate production, with a similar half-maximal effective dose (ED50) around 1 nM. The BN receptor antagonist [Leu13-psi-CH2NH-Leu14]-BN (LLBN) at 1 microM was devoid of agonist activity and displaced the BN dose-response to the right, resulting in a tenfold increase in the ED50 for BN. BN also stimulated a dose-related increase in 45Ca2+ efflux which was also inhibited by LLBN. Two SP analogues [DArg1,D-Pro2,D-Trp7,9,Leu11]-SP and [D-Arg1,D-Phe5,D-Trp7,9,Leu11]-SP ([APheTL]-SP), both antagonized BN-stimulated inositol lipid hydrolysis. [APheTL]-SP (60 and 80 microM) alone also exhibited considerable agonist activity which was not antagonized by LLBN. Indeed, a sub-threshold dose of [APheTL]-SP (40 microM) in the presence of LLBN (10 microM) potentiated the inositol lipid hydrolysis response. BN, GRP, LLBN and [APheTL]-SP all inhibited binding of 125I-labelled GRP to MCF-7 cells, to 50% of that occurring in the absence of the peptides, at concentrations of 150 pM, 150 pM, 150 nM and 600 nM respectively. These data are consistent with the presence of separate but interacting receptors or binding sites for BN and SP analogues, which are coupled to a common signal transduction pathway in human breast cancer cells.

Bombesin receptor from Swiss 3T3 cells Affinity chromatography and reconstitution into phospholipid vesicles

Bombesin and its mammalian counterpart gastrin releasing peptide (GRP) are potent mitogens for Swiss 3T3 cells in which distinct high affinity receptors have been identified. We developed here a probe for specific ligand affinity chromatography by coupling biotin to [lys 1]bombesin. The resulting biotinylated [lys 3]bombesin (BLB) retained biological activity as judged by inhibition of [ 125I]GRP binding to intact cells and membrane preparations and stimulation of rapid Ca 2+ mobilization and DNA synthesis in intact cells. Using this ligand and magnetised beads coated with streptavidin, we extracted differentially a single protein from detergent-solubilized Swiss 3T3 membranes in a BLB-dependent manner. Visualization was achieved either after autoradiograph of metabolically labelled proteins with [ 13S]methionine or by silver staining of larger preparations. In other experiments, elution of BLB-receptor complexes bound to streptavidin beads was carried out at neutral pH and eluted fraction was reconstituted into phospholipid vesicles. This procedure revealed[ 125I]GRP binding activity that exhibited saturability, specificity and a 1946-fold increase in specific activity.

High-Density Functional Gastrin Releasing Peptide Receptors on Primate Cells

Gastrin releasing peptide (GRP) is a 27 amino acid hormone that elicits a variety of biological effects. Receptor-binding antagonists of GRP may have therapeutic use in several pathologic conditions including cancer. The identification and characterization of GRP receptor antagonists have been aided by the use of murine 3T3 cells that possess functional GRP receptors. However, no human or primate cell lines that possess high-density GRP receptors and exhibit a biochemical or biological response to GRP have been described. To address this problem, we examined a series of cell lines and found that GRP specifically binds to Cos-7 monkey cells and stimulates elevation of intracellular calcium in these cells. Cos-7 cells exhibit a single class of high-affinity (dissociation constant = 0.13 nM) GRP binding sites (35,000/cell). Cross-linking experiments that use radiolabeled GRP identified two species of putative GRP receptor proteins (relative molecular mass, 90,000 and 22,000). Competitive binding inhibition studies indicate that Cos-7 cells tightly bind GRP-specific receptor antagonists. These antagonists block the binding of radiolabeled GRP to Cos-7 cells and inhibit GRP-stimulated elevation of intracellular calcium. These properties make Cos-7 cells a useful reagent for the study of GRP receptor antagonists.

Minimal Ligand Analysis of Gastrin‐Releasing Peptide: Receptor Binding and Mitogenesis

Gastrin releasing peptide (GRP) is a peptide hormone containing 27 amino acids which is structurally analogous to the amphibian peptide bombesin. GRP serves a variety of physiological functions and has been implicated in the pathophysiology of small cell lung cancer. Previous work has demonstrated that the modified C terminus of GRP, N-acetyl-GRP-20-27, exerts full agonist activity in a variety of assay systems. However, no systematic comparison of binding of GRP fragments to its receptor and mitogenic potency has been reported. To investigate whether smaller GRP fragments could bind to the GRP receptor without stimulating mitogenesis, we performed binding inhibition and thymidine uptake assays with Swiss 3T3 fibroblasts. These studies were facilitated by the development of a novel tritiated GRP-based radioligand, [3H-Phe15] GRP-15-27, which exhibits enhanced chemical stability compared to iodinated GRP derivatives. We examined a series of C-terminal GRP fragments, from the pentapeptide to the octapeptide, with both N-acetyl and free amine moieties at the N terminus. N-Acetylated derivatives were more potent than their primary amine counterparts in both assays. Deletion of N-terminal residues from GRP-20-27 resulted in significant loss of potency in both assays: the EC50 values of N-acetyl-GRP-21-27 were 10(2)-fold higher than N-acetyl-GRP-20-27, those of N-acetyl-GRP-22-27 were 10(4)-fold higher, and N-acetyl-GRP-23-27 showed minimal activity at concentrations below 100 microM. These results suggest that 1) both His20 and Trp21 play an important role in binding of GRP to the receptor, and 2) for this series of N-terminal deletions, binding to the receptor and mitogenic activity are tightly coupled.

Minimal ligand analysis of gastrin releasing peptide. Receptor binding and mitogenesis.

Gastrin releasing peptide (GRP) is a peptide hormone containing 27 amino acids which is structurally analogous to the amphibian peptide bombesin. GRP serves a variety of physiological functions and has been implicated in the pathophysiology of small cell lung cancer. Previous work has demonstrated that the modified C terminus of GRP, N-acetyl-GRP-20-27, exerts full agonist activity in a variety of assay systems. However, no systematic comparison of binding of GRP fragments to its receptor and mitogenic potency has been reported. To investigate whether smaller GRP fragments could bind to the GRP receptor without stimulating mitogenesis, we performed binding inhibition and thymidine uptake assays with Swiss 3T3 fibroblasts. These studies were facilitated by the development of a novel tritiated GRP-based radioligand, [3H-Phe15] GRP-15-27, which exhibits enhanced chemical stability compared to iodinated GRP derivatives. We examined a series of C-terminal GRP fragments, from the pentapeptide to the octapeptide, with both N-acetyl and free amine moieties at the N terminus. N-Acetylated derivatives were more potent than their primary amine counterparts in both assays. Deletion of N-terminal residues from GRP-20-27 resulted in significant loss of potency in both assays: the EC50 values of N-acetyl-GRP-21-27 were 10(2)-fold higher than N-acetyl-GRP-20-27, those of N-acetyl-GRP-22-27 were 10(4)-fold higher, and N-acetyl-GRP-23-27 showed minimal activity at concentrations below 100 microM. These results suggest that 1) both His20 and Trp21 play an important role in binding of GRP to the receptor, and 2) for this series of N-terminal deletions, binding to the receptor and mitogenic activity are tightly coupled.