A substance P antagonist also inhibits specific binding and mitogenic effects of vasopressin and bombesin-related peptides in Swiss 3T3 cells

Pharmacological lineage analysis revealed the binding affinity of broad-spectrum substance P antagonists to receptors for gonadotropin-releasing peptide

A group of synthetic substance P (SP) antagonists, such as [Arg(6),D-Trp(7,9),N(Me)Phe(8)]-substance P(6-11) and [D-Arg(1),D-Phe(5),D-Trp(7,9),Leu(11)]-substance P, bind to a range of distinct G-protein-coupled receptor (GPCR) family members, including V1a vasopressin receptors, and they competitively inhibit agonist binding. This extended accessibility enabled us to identify a GPCR subset with a partially conserved binding site structure. By combining pharmacological data and amino acid sequence homology matrices, a pharmacological lineage of GPCRs that are sensitive to these two SP antagonists was constructed. We found that sensitivity to the SP antagonists was not limited to the Gq-protein-coupled V1a and V1b receptors; Gs-coupled V2 receptors and oxytocin receptors, which couple with both Gq and Gi, also demonstrated sensitivity. Unexpectedly, a dendrogram based on the amino acid sequences of 222 known GPCRs showed that a group of receptors sensitive to the SP antagonists are located in close proximity to vasopressin/oxytocin receptors. Gonadotropin-releasing peptide receptors, located near the vasopressin receptors in the dendrogram, were also sensitive to the SP analogs, whereas α1B adrenergic receptors, located more distantly from the vasopressin receptors, were not sensitive. Our finding suggests that pharmacological lineage analysis is useful in selecting subsets of candidate receptors that contain a conserved binding site for a ligand with broad-spectrum binding abilities. The knowledge that the binding site of the two broad-spectrum SP analogs partially overlaps with that of distinct peptide agonists is valuable for understanding the specificity/broadness of peptide ligands.

Anticancer activity of a peptide combination in gastrointestinal cancers targeting multiple neuropeptide receptors

A novel peptide combination consisting of four synthetic neuropeptide analogs of Vasoactive Intestinal Peptide (VIP), Bombesin, Substance P and Somatostatin has been found to have potent anticancer activity in vitro and in vivo. The receptors of these four neuropeptides are known to be over expressed in various cancers. We have found the presence of native neuropeptides in the culture supernatant of the primary tumor cells of human colon adenocarcinomas. It was further demonstrated by receptor-ligand assays that not only do these tumor cells synthesize and secrete four peptide hormones but also possess specific high affinity receptors on their surface. Screening a large panel of analogs to the four peptide hormones on tumor cell proliferation led to the identification of four cytotoxic analogs, the combination of which was code-named DRF7295. The design and synthesis of the peptide analogs have been described in this paper. In vitro anticancer activity of DRF7295 was studied in a large panel of human tumor cells. Gastrointestinal tumor cells of the colon, pancreas and duodenum were found to be most sensitive to DRF7295 with moderate activity seen in glioblastoma, prostate, leukemia and those of oral cancer cells. Efficacy studies in xenograft models of colon and duodenum resulted in T/C% of less than 40%, which is indicative of strong tumor regressing potential of DRF7295 in gastrointestinal cancers. Acute and long-term toxicity studies as well as safety pharmacology studies conducted indicate the safety of the drug upon systemic administration with no significant adverse pharmacological effects.

Characterization of the bombesin-like peptide receptor family in primates

In mammals, bombesin-like peptides mediate a broad range of physiological functions through binding to three highly conserved G-protein-coupled receptors: the neuromedin B-preferring, the gastrin-releasing peptide-preferring, and the bombesin-receptor subtype 3. Selective modulation of these receptors presents opportunities for the development of novel therapeutics. To ascertain if rhesus monkey could serve as a surrogate animal model for the development of modulators of bombesin-like receptor function, we undertook a search for additional receptor family members and studied the expression profiles of the three known bombesin-related receptors. We found no evidence for additional receptor family members in mammals, suggesting that the expression of the previously described bombesin-receptor subtype 4 is limited to amphibians. We studied the distribution of the three receptors in a broad array of human and rhesus monkey tissues. Based on the similarity between the human and the rhesus expression profiles, we conclude that the rhesus monkey may be a suitable animal model to evaluate the clinical efficacy and potential side effects of bombesin-like peptide ligands.

Nonpeptide neuromedin B receptor antagonists inhibit the proliferation of C6 cells

The ability of nonpeptide antagonists to interact with neuromedin B receptors on C6 cells was investigated. 2-[3-(2, 6-Diisopropyl-phenyl)-ureido]3-(1H-indol-3-yl)-2-methyl-N-(1-pyridin- 2-yl-cyclohexylmethyl)-proprionate (PD165929), 3-(1H-indol-3-yl)-2-methyl-2-[3(4-nitro-phenyl)-ureido]-N-(1-pyridin- 2-yl-cyclohexylmethyl)-propionamide (PD168368) and 3-(1H-indol-3-yl)-N-[1-(5-methoxy-pyridin-2-yl)-cyclohexylmethyl]- 2-m ethyl-2-[3-(4-nitro-phenyl)-ureido]-propionamide (PD176252) inhibited (125I-Tyr0)neuromedin B binding with IC50 values of 2000, 40 and 50 nM, respectively. Because neuromedin B is a G-protein coupled serpentine receptor, the effects of neuromedin B antagonists on second messenger production and proliferation were investigated. PD168368 inhibited the ability of 10 nM neuromedin B to cause elevation of cytosolic Ca2+, whereas it had no effect on basal cytosolic Ca2+. PD168368 inhibited the ability of 100 nM neuromedin B to cause elevation of c-fos mRNA. Also, PD168368 in a dose-dependent manner inhibited the ability of 100 nM neuromedin B to cause phosphorylation of focal adhesion kinase. Using a [3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide] assay, the order of antagonist potency to inhibit C6 proliferation was PD168368=PD176252>PD165929. Also, 1 microM PD168368 and PD176252 significantly inhibited colony number using a proliferation assay in vitro. PD168368 significantly inhibited C6 xenograft growth in nude mice in vivo. These results indicate that PD168368 is a C6 cell neuromedin B receptor antagonist, which inhibits proliferation.

Vasopressin accelerates protein synthesis in neonatal rat cardiomyocytes

Arginine vasopressin (AVP) has been shown to promote vascular smooth muscle cell hypertrophy and hyperplasia of fibroblasts. The present study examines the effect of AVP and endothelin-1 (ET-1) on protein, DNA, and RNA synthesis in primary cultures of serum deprived neonatal rat cardiomyocytes (RC) as assessed by changes in [3H] phenylalanine, [3H] thymidine, and [14C] uridine incorporation respectively. Both AVP and ET-1 evoked significant increases in protein synthesis in RC of 36 +/- 12% (p < 0.05) and 53 +/- 22% (p < 0.01) respectively. The stimulating action of AVP on [3H] phenylalanine incorporation was abolished by pretreatment with 2-nitro-4carboxyphenyl-N, N-diphenylcarbamate (NCDC), a phospholipase C (PLC) inhibitor. [14C] uridine incorporation was significantly higher in cells incubated with ET-1 (95 +/- 12%) but not AVP (9 +/- 11%). Neither AVP nor ET-1 significantly affected cell number or [3H]thymidine incorporation, suggesting a lack of a hyperplastic effect. AVP evoked an increase in [Ca2+]i levels (162 +/- 12 nmol/L from a basal value of 77 +/- 6 nmol/L) which was completely abolished by pretreatment with either NCDC or cyclopiazonic acid (sarcoplasmic reticulum (SR) Ca2+ pump inhibitor) but unaffected by ryanodine (ryanodine sensitive SR Ca2+ store depletor). Taken together, these data suggest that AVP, in a PLC dependent manner, both stimulates protein synthesis and augments [Ca2+]i release in RC from ryanodine insensitive (IP3 sensitive) Ca2+ stores. Thus, AVP may promote cardiac hypertrophy via direct effects on cardiomyocyte protein synthesis secondary to IP3 mediated [Ca2+]i release.

Synthesis of peptide and pseudopeptide amides inhibiting the proliferation of small cell and epithelial types of lung carcinoma cells

Small cell lung cancer (SCLC) cell lines produce and secrete various peptide hormones, e.g. bombesin (BN)/gastrin releasing peptide (GRP) like peptides that are proposed to function as their autocrine growth factors. To inhibit the proliferative effect of these hormones we have synthesized short chain BN[7-14]-analogues replacing the C-terminal peptide bond by a methylene-amino (-CH2NH-) unit and introducing D-Phe or D-Ser into position 12. As several substance P (SP) analogues were found to inhibit the growth of SCLC cells, some short chain SP-analogues have been synthesized. (Pseudo)octapeptides were synthesized in solution, by fragment condensation using the DCC/HOPfp method. Fragments and SP-analogues were synthesized stepwise using pentafluorophenyl esters. The resistance to hydrolysis of the reduced peptide bond made permitted exact quantification of the Leupsi(CH2NH)Leu pseudopeptide in hydrolysates. The binding ability of both types of peptides to BN-receptors on Swiss 3T3 mouse fibroblast cells and their antiproliferative effect on NCI-H69 human SCLC cell line have been tested and compared with a short chain SP-antagonist pHOPA-D-Trp-Phe-D-Trp-Leu-Leu-NH2 (R) previously described as a potent inhibitor of SCLC proliferation. While BN-analogues showed weak activity in inhibition of proliferation of SCLC cells, SP-analogues 6: D-MePhe-D-Trp-Phe-D-Trp-Leu(psi)(CH2NH)-Leu-NH2 and 7: D-MePhe-DTrp-Phe-D-Trp-Leu-MPA, in spite of greatly diminished affinity towards the BN-receptor, inhibited SCLC proliferation more effectively than R (6: IC50 = 2 microM, 7: IC50 = 5 microM and R: IC50 = 10 microM). Moreover, 6 inhibited the respiratory activity of SK-MES 1 epithelial type of lung carcinoma cells in proliferating but not in the quiescent state, suggesting that the antiproliferative effect of these compounds is not due to simple cytotoxicity. These short chain analogues of SP might be promising candidates as therapeutic agents in the treatment of SCLC.

Neuromedin B stimulates arachidonic acid release, c-fos gene expression, and the growth of C6 glioma cells

The effects of neuromedin B (NMB) on C6 glioma cells were investigated. NMB bound with high affinity (IC50 = 1 nM) to C6 cells whereas BN and GRP were less potent (IC50 = 40 and 100 nM). NMB (1 nM) elevated cytosolic Ca2+ in individual C6 cells and the increase in cytosolic Ca2+ was reversed by 1 microM [D-Arg1, D-Pro2,D-Trp7.9,Leu11]substance P [APTTL]SP, a broad spectrum antagonist. NMB stimulated [3H]arachidonic acid release from C6 cells and the increase in [3H]arachidonic acid release was reversed [APTTL]SP. NMB increased transiently c-fos gene expression in C6 cells. NMB increased the number of C6 colonies in soft agar and the increase in growth caused by NMB was reversed by [APTTL]SP. These data suggest that NMB receptors may regulate the proliferation of C6 cells.

Effect of a new, potent, non-peptide V1a vasopressin antagonist, SR 49059, on the binding and the mitogenic activity of vasopressin on Swiss 3T3 cells

The effects of SR 49059, a new non-peptide, selective arginine vasopressin (AVP), V1a antagonist, were investigated both on AVP's receptors and on the mitogenic effects of AVP on Swiss 3T3 fibroblasts. We characterized the AVP V1a receptors on Swiss 3T3 cell membranes using the new highly specific AVP V1a radioiodinated ligand, 125I-linear AVP antagonist. Specific binding of the 125I-linear AVP antagonist was saturable, time-dependent and reversible. A single class of high affinity binding sites was identified with an apparent Kd of 40 +/- 20 pM and a Bmax of 63 +/- 20 fmol/mg protein. 125I-Linear AVP antagonist binding to its receptors was potently inhibited in a concentration-dependent manner by AVP, by the peptide V1a antagonist d(CH2)5Tyr(Me)AVP and by the synthetic V1a antagonist, SR 49059 (IC50 in the nanomolar range) while OPC-21268, another non-peptide compound, was about 100-fold less potent. Both DDAVP, a selective V2 agonist, and oxytocin exhibited low affinity (IC50 > 1 microM) in agreement with the AVP V1a nature of the site identified on Swiss 3T3 cells. In addition, the broad-spectrum antiproliferative agent [Arg6, D-Trp7,9, MePhe8] substance P (6-11), was also able to interact at 3T3 AVP V1a receptors (IC50 = 395 +/- 170 nM). The mitogenic effects of AVP on quiescent Swiss 3T3 cells, assessed through [3H]thymidine incorporation, were selectively, stereospecifically and strongly inhibited by SR 40959 (IC50 = 14 +/- 2 nM) while OPC-21268 was inactive up to 220 nM. SR 49059 was even about six times more efficient than d(CH2)5Tyr(Me)AVP in inhibiting AVP-induced DNA synthesis. Moreover, SR 49059 fully inhibited Swiss 3T3 fibroblast proliferation since it completely blocked AVP-stimulated 3T3 cell growth from the G1/G0 into the S/G2M phase, as evidenced by cell cycle analysis using a cytofluorometer. In summary, SR 49059, through direct interaction at AVP V1a receptors, exerts the most potent antiproliferative effect yet described for any V1a antagonist on Swiss 3T3 cells.

Neurohypophyseal peptides as regulators of growth and development. A review

In addition to their classical hormonal role, the neurohypophyseal peptides vasopressin (AVP) and oxytocin (OT) are also implicated as regulators of growth and development. Mitogenic actions of AVP are particularly well characterized and may underly the potential role of AVP as an autocrine regulator of tumor growth. Effects of AVP and OT on neural development are suggested by numerous studies, but definitive physiological evidence is lacking. Current studies on the molecular characterization of AVP and OT receptors, and on transgenic animals will provide insights into the developmental actions of neurohypophyseal peptides.

Evidence of NK1 and NK2 tachykinin receptors and their involvement in histamine release in a murine mast cell line

Binding of [3H]substance P (SP) and histamine release were examined using a cloned mouse mast cell line. SP binding was saturable and specific. In the presence of 30 mM Na2SO4/50 mM Tris buffer, SP interacted with two types of binding sites with Kd values of 0.3 and 40 nM. High-affinity SP binding was blocked by the inclusion of 0.5 uM of the NK1 receptor selective ligand septide in the binding mixture. Neurokinin A (NKA) evoked concentration-dependent histamine release. At concentrations in the nanomolar range, the NK1 preferring agonists SP, SP methylester and physalaemin evoked less than or equal to 5% net release of histamine, which was substantially less than the maximum effect of NKA (+37%) in the micromolar range. Pretreatment of the cells with the NK2 antagonist peptide A reduced NKA-induced histamine release. [D-Arg1,D-Phe5,D-Trp7,9,Leu11]-substance P, a putative SP antagonist, also elicited histamine release in the micromolar range, apparently acting as an agonist at the NK2 site. Compound 48/80, N-terminal SP fragments, neurokinin B and the two selective NK2 receptor antagonists cyclo(Gln-Trp-Phe-(R)-[ANC-2]Leu-Met) (peptide A) and cyclo(Gln-Trp-Phe-Gly-Leu-Met) (peptide B) were ineffective. Although the results suggest the coexistence of functional NK1 and NK2 receptors, it appears that in this mast cell line neurokinin-induced histamine release is primarily mediated by the NK2 receptor, characterized biochemically as a low affinity binding site with a Kd value of 40 nM for SP.

Vasopressin elevates cytosolic calcium in small cell lung cancer cells

The ability of vasopressin to elevate cytosolic Ca2+ in small cell lung cancer (SCLC) cells was investigated. Ten nanomolar vasopressin elevated the cytosolic Ca2+ in 6 of 8 SCLC cell lines that were loaded with Fura-2 AM. Using SCLC cell line NCI-H345, the effect of vasopressin was dose dependent, being maximal at 100 nM, where the cytosolic Ca2+ was elevated from 150 to 210 nM. Because addition of 1 mM EGTA had no effect on the vasopressin response, vasopressin released Ca2+ from intracellular pools. Also, oxytocin weakly elevated the cytosolic Ca2+. The response to vasopressin was strongly blocked by [(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)1,O-MeTyr2,Arg8]vasopressin and weakly blocked by [(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)1,O-MeTyr2,Orn8]vasotocin. These data suggest that V1 vasopressin receptors are present on SCLC cells.

[D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P, a neuropeptide antagonist, blocks binding, Ca2(+)-mobilizing, and mitogenic effects of endothelin and vasoactive intestinal contractor in mouse 3T3 cells

Endothelin (ET1) and vasoactive intestinal contractor (VIC) stimulate quiescent Swiss 3T3 cells to resume DNA synthesis acting synergistically with epidermal growth factors (EGF) and other mitogens. The peptide [D-Arg1,D-Phe5,D-Trp7,9,Leu11] substance P has been identified as a broad spectrum neuropeptide antagonist which blocks the binding and biological effects of the Ca2(+)-mobilizing neuropeptides bombesin, vasopressin, and bradykinin. In the present study we show that [D-Arg1,D-Phe5,D-Trp7,9,Leu11] substance P also acts as an ET1/VIC antagonist as judged by the following criteria: a) inhibition of specific 125I-labelled ET1 binding to a ET1/VIC receptor in a competitive and dose-dependent manner; b) blocking of the rapid increase in the cytosolic Ca2+ concentration promoted by ET1 or VIC; and c) inhibition of DNA synthesis stimulated by VIC in the presence of EGF. The inhibitory effects of [D-Arg1,D-Phe5,D-Trp7,9,Leu 11] substance P on Ca2+ mobilization and DNA synthesis were reversed by increasing the concentration of VIC. This is the first time that a peptide structurally unrelated to ET1 or VIC is shown to block the binding and mitogenic effects of peptides of the endothelin family.

Neuropeptide stimulation of calcium flux in human lung cancer cells: delineation of alternative pathways

Calcium ion flux following the administration of a series of neuropeptides, N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate, and serum was monitored by flow cytometry in selected lung and breast cancer cell lines and Chinese hamster ovary cell line CHO-K1. Calcium ion flux was monitored in individual cells by flow cytometry using the indicator indo-1 AM. Five groups of neuropeptides produced calcium flux changes in lung cancer cell lines and CHO-K1 cells but not in breast cancer cells. The peak increase in free calcium was reached within 10 sec of peptide administration and declined to resting levels in 70-120 sec. When two or more members of the same group were administered simultaneously, calcium flux changes were identical to that produced by each single peptide. When two or more members of different groups were administered simultaneously, an increased calcium release occurred. When identical peptides or peptides from the same group were administered sequentially after the return of calcium concentrations to resting values, no calcium flux resulted from the second peptide. When peptides from different active groups were administered sequentially, a new calcium flux occurred after each peptide. These data are interpreted to mean that members of each active group of peptides trigger a different calcium flux pathway. Thus, many such pathways and different metabolic states exist within the cell. Elucidation of calcium flux pathways in normal and cancer cells may lead to greater understanding of the nature of the malignant defect.

Bombesin receptor in membranes from Swiss 3T3 cells. Binding characteristics, affinity labelling and modulation by guanine nucleotides

Bombesin-like neuropeptides, including mammalian gastrin-releasing peptide (GRP), are potent mitogens for Swiss 3T3 cells. In this study, we have characterized the bombesin receptor in membrane preparations from these cells. Addition of Mg2+ during cell homogenization was essential to preserve 125I-GRP binding activity in the resulting membrane preparation. The effect of Mg2+ was concentration dependent, with a maximum at 5 mM. Specific binding of 125I-GRP was saturable; Scatchard analysis indicated a single class of high-affinity sites of Kd = (2.1 +/- 0.3) x 10(-10) M at 15 degrees C and Kd = (1.9 +/- 0.4) x 10(-10) M at 37 degrees C, and a maximum binding capacity of 580 +/- 50 fmol/mg of protein (15 degrees C) or 604 +/- 40 fmol/mg of protein (37 degrees C). The kinetically derived dissociation constant was 1.5 x 10(-10) M. 125I-GRP binding was inhibited in a concentration-dependent manner by various peptides containing the highly conserved C-terminal heptapeptide of the bombesin family, including bombesin, GRP, neuromedin B and the 8-14 fragment of bombesin. In contrast, a variety of structurally unrelated mitogens and neuropeptides had no effect. The cross-linking agent ethyleneglycolbis(succinimidylsuccinate) covalently linked 125I-GRP to a single Mr 75 000-85 000 protein in membrane preparations of 3T3 cells. Affinity labelling of this molecule was specific and dependent on the presence of Mg2+ during membrane preparation. Finally, the non-hydrolysable GTP analogue guanosine-5'-[gamma-thio]triphosphate (GTP[S]) caused a concentration-dependent inhibition of 125I-GRP binding and cross-linking to 3T3 cell membranes [concentration giving half-maximal inhibition (IC50) approximately 0.2 microM]. The inhibitory effect was specific (GMP, ATP or ATP[S] had no effect at 10 microM) and was due to an increase in Kd from (1.7 +/- 0.2) x 10(-10) M to (4.3 +/- 0.6) x 10(-10) M in the presence of 10 microM-GTP[S]. This modulation of ligand affinity and cross-linking implies that the bombesin receptors that mediate mitogenesis in Swiss 3T3 cells are coupled to a guanine-nucleotide-binding-protein signal-transduction pathway.

Heterologous desensitization of bombesin-induced mitogenesis by prolonged exposure to vasopressin: a post-receptor signal transduction block

Prolonged exposure of quiescent Swiss 3T3 cells to vasopressin prevents mitogenic stimulation on subsequent addition of bombesin. This heterologous desensitization is selective and can be mimicked by vasopressin agonists, including [Lys8]vasopressin and oxytocin but not by the V1-type-specific vasopressin receptor antagonist [Pmp1,O-Me-Tyr2,Arg8]vasopressin [where Pmp is 1-(beta-mercapto-beta,beta-cyclopenthamethylene propionic acid)]. Furthermore, vasopressin-induced loss of responsiveness to bombesin can be blocked by addition of this antagonist, indicating that heterologous desensitization is mediated through the vasopressin receptor. Desensitization requires prolonged incubation (half-maximal desensitization occurring after approximately 20 hr of pretreatment) and continuous protein synthesis. Bombesin responsiveness is restored by incubation in the absence of vasopressin. Pretreatment does not alter the number, affinity, or internalization capacity of the bombesin receptors. However, the induction of the protooncogene c-fos by bombesin is profoundly inhibited after vasopressin pretreatment. We suggest that the coupling of the activated bombesin receptor to the generation of its early signals is impaired in desensitized cells.

N-isobutyryl-His-Trp-Ala-Val-D-Ala-His-Leu-NHMe (ICI 216140) a potent in vivo antaconist analogue of bombesin/gastrin releasing peptide (BN/GRP) derived from the C-terminal sequence lacking the final methionine residue

The GRP receptor mediated growth response in Swiss 3T3 cells has been used to identify BN/GRP antagonists. Analysis of bombesin antagonism by substance P analogues and by truncated GRP analogues revealed that deletion of the C-terminal methionine residue was important for antagonism. Des-Met analogues showing potent antagonist activity in the in vitro 3T3 system (IC50 approximately 2nM) were synthesized. Further structural modification of these peptides led to the identification of (CH3)2CHCO-His-Trp-Ala-Val-D-Ala-His-Leu-NHCH3 (ICI 216140) which reduced bombesin-stimulated rat pancreatic amylase secretion to basal levels when administered subcutaneously at 2.0 mg per kg.

Substance P analogues function as bombesin receptor antagonists and inhibit small cell lung cancer clonal growth

Human small cell lung cancer (SCLC) produces and secretes BN/GRP (bombesin/gastrin releasing peptide). Because BN stimulates the growth of SCLC cells and these cells have receptors for BN-like peptides, it is important to define agents which disrupt this self-promoting autocrine growth cycle. Here, substance P analogues were evaluated as BN receptor antagonists using SCLC cell lines. (D-Arg1, D-Pro2, D-Trp7.9, Leu11) substance P [(APTTL)SP] was one of the more potent analogues tested in inhibiting BN-like peptide receptor binding with an IC50 value of 1 microM. Micromolar concentrations of (APTTL)SP antagonized BN receptor mediated elevation of cytosolic Ca2+ levels and decreased the colony formation in soft agarose. These data suggest that SP analogues function as SCLC BN receptor antagonists and may be useful in disrupting the autocrine growth function of BN-like peptides.

[Leu13-psi(CH2NH)Leu14]bombesin is a specific bombesin receptor antagonist in Swiss 3T3 cells

The pseudopeptide [Leu13-psi(CH2NH)Leu14]bombesin blocks bombesin-stimulated mitogenesis in Swiss 3T3 cells in a competitive and reversible manner, but not that of other mitogens. It inhibits the mobilization of intracellular Ca2+ and activation of protein kinase C by bombesin-like peptides. It acts at receptor level, as shown by inhibition of [125I]GRP binding and reduction in cross-linking of the Mr 75-85,000 receptor-associated protein. Thus [Leu13-psi(CH2NH)Leu14]bombesin is a specific bombesin receptor antagonist in Swiss 3T3 cells which blocks long-term growth promoting effects of bombesin-like peptides.

Bombesin and bombesin antagonists: studies in Swiss 3T3 cells and human small cell lung cancer

Bombesins are potent growth factors for murine Swiss 3T3 cells. Using these cells in chemically defined conditions we have been able to characterise the bombesin receptor and the early signals preceding DNA synthesis. We describe two substance P analogues [DArg1, DPro2, DTrp7,9, Leu11] substance P and [DArg1, DPhe5, DTrp7,9, Leu11] substance P which competitively block the binding of bombesins to their receptor and all the events leading to mitogenesis. Bombesins are secreted by human small cell lung cancers (SCLC) and may act as autocrine growth factors for these tumours, so the development of peptide bombesin antagonists could have therapeutic implications. We demonstrate that the antagonists can reversibly inhibit the growth of SCLC in vitro, with relatively little effect on other lung tumours.

[D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P, a potent bombesin antagonist in murine Swiss 3T3 cells, inhibits the growth of human small cell lung cancer cells in vitro

In the search for a more potent bombesin antagonist, we found [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P to be effective in mouse fibroblasts and to inhibit the growth of small cell lung cancer, a tumor that secretes bombesin-like peptides that may act as autocrine growth factors. In murine Swiss 3T3 cells, [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P proved to be a bombesin antagonist as judged by the following criteria: (i) inhibition of DNA synthesis induced by gastrin-releasing peptide and other bombesin-like peptides; (ii) inhibition of 125I-labeled gastrin-releasing peptide binding to the bombesin/gastrin-releasing peptide receptor; (iii) reduction in cross-linking of the Mr 75,000-85,000 protein putatively a component of the bombesin/gastrin-releasing peptide receptor; (iv) blocking of early cellular events that precede mitogenesis--calcium mobilization and inhibition of epidermal growth factor binding. [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P was 5-fold more potent than the antagonist [D-Arg1,D-Pro2,D-Trp7,9,Leu11]substance P. [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P also inhibits mitogenesis induced by vasopressin but not that induced by a variety of other mitogens. Both antagonists reversibly inhibited the growth of small cell lung cancer in vitro in a concentration-dependent manner. Peptide antagonists could, therefore, have far-reaching therapeutic implications.

Two classes of antagonist interact with receptors for the mitogenic neuropeptides bombesin, bradykinin, and vasopressin

While screening neuropeptides for activity as growth factors we have found that bradykinin is a mitogen for Swiss 3T3 cells. It acts synergistically with insulin, and maximal effect is obtained at 10 nM. It acts through a distinct receptor, characterized as a B2 subtype using bradykinin analogues. The neuropeptides bombesin and vasopressin are also potent mitogens for Swiss 3T3 cells. The substance P antagonists [DArg1, DPro2, DTrp7,9, Leu11] substance P and [DArg1, DPhe5, DTrp7,9, Leu11]substance P are inhibitors of DNA synthesis stimulated by both bombesin and vasopressin. In the present study they were found also to inhibit bradykinin-induced mitogenesis. In contrast, the ligand-specific antagonists [Leu13-psi(CH2NH)Leu14]bombesin, [Pmp1, OMeTyr2, Arg8]vasopressin and [DArg0, Hyp3, Thi5,8, DPhe7]bradykinin showed no cross-inhibition with each others receptors. We propose therefore that the receptors for the mitogenic neuropeptides bombesin, vasopressin, and bradykinin can interact with two classes of antagonist, one recognizing the ligand binding site (e.g., [Leu13-psi(CH2NH)Leu14]bombesin) and the other recognizing a common domain shared by the three receptors (e.g., [DArg1, DPhe5, DTrp7,9, Leu11]substance P).

Rapid dephosphorylation of a Mr 80,000 protein, a specific substrate of protein kinase C upon removal of phorbol esters, bombesin and vasopressin

The mitogens phorbol 12,13-dibutyrate, bombesin and vasopressin stimulate the phosphorylation of an acidic Mr 80,000 cellular protein, a specific substrate of protein kinase C, in intact Swiss 3T3 cells. Phosphorylation of this substrate was rapidly reversed upon the removal of each of these agents. Dephosphorylation occurred with a similar half-life in each of the cases studied (2.2, 1.5 and 2 minutes for phorbol 12,13-dibutyrate, bombesin and vasopressin respectively) and agreed closely with the dissociation of the ligands from their specific high-affinity binding sites in Swiss 3T3 cells.

Early signals in the mitogenic response

Polypeptide growth factors, regulatory peptides, and a variety of pharmacological agents acting alone or synergistically induce mitogenesis in cultured fibroblasts. The early signals in the membrane, cytosol, and nucleus promoted by these extracellular factors, together with their mitogenic effectiveness, are integrated in a unified hypothesis for the regulation of fibroblast growth.