Ability of various bombesin receptor agonists and antagonists to alter intracellular signaling of the human orphan receptor BRS-3

The Nonpeptide Agonist MK-5046 Functions As an Allosteric Agonist for the Bombesin Receptor Subtype-3

Allosteric ligands of various G-protein-coupled receptors are being increasingly described and are providing important advances in the development of ligands with novel selectivity and efficacy. These unusual properties allow expanded opportunities for pharmacologic studies and treatment. Unfortunately, no allosteric ligands are yet described for the bombesin receptor family (BnRs), which are proposed to be involved in numerous physiologic/pathophysiological processes in both the central nervous system and peripheral tissues. In this study, we investigate the possibility that the bombesin receptor subtype-3 (BRS-3) specific nonpeptide receptor agonist MK-5046 [(2S)-1,1,1-trifluoro-2-[4-(1H-pyrazol-1-yl)phenyl]-3-(4-[[1-(trifluoromethyl)cyclopropyl]methyl]-1H-imidazol-2-yl)propan-2-ol] functions as a BRS-3 allosteric receptor ligand. We find that in BRS-3 cells, MK-5046 only partially inhibits iodine-125 radionuclide (125I)-Bantag-1 [Boc-Phe-His-4-amino-5-cyclohexyl-2,4,5-trideoxypentonyl-Leu-(3-dimethylamino) benzylamide N-methylammonium trifluoroacetate] binding and that both peptide-1 (a universal BnR-agonist) and MK-5046 activate phospholipase C; however, the specific BRS-3 peptide antagonist Bantag-1 inhibits the action of peptide-1 competitively, whereas for MK-5046 the inhibition is noncompetitive and yields a curvilinear Schild plot. Furthermore, MK-5046 shows other allosteric behaviors, including slowing dissociation of the BRS-3 receptor ligand 125I-Bantag-1, dose-inhibition curves being markedly affected by increasing ligand concentration, and MK-5046 leftward shifting the peptide-1 agonist dose-response curve. Lastly, receptor chimeric studies and site-directed mutagenesis provide evidence that MK-5046 and Bantag-1 have different binding sites determining their receptor high affinity/selectivity. These results provide evidence that MK-5046 is functioning as an allosteric agonist at the BRS-3 receptor, which is the first allosteric ligand described for this family of receptors. SIGNIFICANCE STATEMENT: G-protein-coupled receptor allosteric ligands providing higher selectivity, selective efficacy, and safety that cannot be obtained using usual orthosteric receptor-based strategies are being increasingly described, resulting in enhanced usefulness in exploring receptor function and in treatment. No allosteric ligands exist for any of the mammalian bombesin receptor (BnR) family. Here we provide evidence for the first such example of a BnR allosteric ligand by showing that MK-5046, a nonpeptide agonist for bombesin receptor subtype-3, is functioning as an allosteric agonist.

Discovery of oridonin as a novel agonist for BRS-3

BACKGROUND

Bombesin Receptor Subtype-3 (BRS-3, Bombesin-like receptor, BB₃) is an orphan G-protein coupled receptor (GPCR). Recent studies have shown that BRS-3 played a vital role in glucose regulation, insulin secretion, and energy homeostasis. Therefore, discovering more novel exogenous ligands with diverse structures for BRS-3 will be of great importance for target validation and drug development.

PURPOSE

In this study, we aim to discover new agonists of BRS-3 from our natural compound libraries, providing a new probe to study the function of BRS-3.

STUDY DESIGN

Multiple cell-based assays and in vivo experiments were performed to identify the new ligand.

METHODS

BRS-3 overexpression cells were coupled with FLIPR assay, homogeneous time-resolved fluorescence (HTRF) IP-ONE assay, dynamic mass redistribution (DMR) assay, β-arrestin2 recruitment assay, and western blot to determine receptor activation and downstream signaling events. To further validate the target of BRS-3, a series of in vitro and in vivo experiences were conducted, including glucose uptake, glucose transporter type 4 (GLUT4) transportation in C2C12, and oral glucose tolerance test (OGTT) in mice.

RESULTS

We discovered and identified oridonin as a novel small molecule agonist of BRS-3, with a moderate affinity (EC₅₀ of 2.236 × 10^-7 M in calcium mobilization assay), specificity, and subtype selectivity. Further in vitro and in vivo tests demonstrated that oridonin exerted beneficial effects in glucose homeostasis through activating BRS-3.

CONCLUSIONS

Oridonin, as the discovered new ligand of BRS-3, provides a valuable tool compound to investigate BRS-3's function, especially for target validation in type 2 diabetes and obesity. Oridonin is promising as a lead compound in the treatment of metabolic disorders. Compared to the known agonists of BRS-3, we can take advantage of the multiple reported pharmacological activities of ODN as a natural product and assess whether these pharmacological activities are regulated by BRS-3. This may facilitate the discovery of novel functions of BRS-3.

Bombyx neuropeptide G protein-coupled receptor A14 and A15 are two functional G protein-coupled receptors for CCHamide neuropeptides

CCHamides are newly identified insect neuropeptides, which are widely occurring in most insects. However, our knowledge about their signaling characteristics and physiological roles is still limited. Here, we cloned two full-length cDNAs encoding putative CCHamide receptors, Bombyx neuropeptide GPCR A14 (BNGR-A14) and -A15 (BNGR-A15), from the brain of B. mori larvae. Characterization of signaling indicated that Bombyx CCHamide-1 and CCHamide-2 are specific endogenous ligands for BNGR-A15 and BNGR-A14, respectively. Further functional assays combined with specific inhibitors demonstrated that upon activation by CCHamide-2, BNGR-A14 elicited significant increases in CRE-driven luciferase activity, intracellular Ca2+ mobilization and ERK1/2 phosphorylation in a Gq inhibitor-sensitive manner, while BNGR-A15 was activated by CCHamide-1, thus leading to intracellular accumulation of cAMP, Ca2+ mobilization, and ERK1/2 phosphorylation in a Gs and Gq inhibitor-sensitive manner. Based on these findings, we designated the receptors BNGR-A15 and -A14 as Bommo-CCHaR-1 and -2, respectively. In addition, our results showed that CCHamides are considered to require intrachain disulfide bonds to activate their respective receptor in the physiological concentration range. Moreover, quantitative RT-PCR analysis revealed that CCHamide-1 is more likely to serve as a brain-gut peptide to regulate feeding behavior and growth through BNGR-A15, whereas the CCHamide-2 signaling system might play an important role in the control of multiple physiological processes. Our findings provide in-depth information on CCHamide-1 and -2-mediated signaling, facilitating further elucidation of their endocrinological roles in the regulation of fundamental physiological processes.

5-Azacytidine promotes shoot regeneration during Agrobacterium-mediated soybean transformation

Agrobacterium-mediated soybean transformation has been greatly improved in recent years, however the transformation efficiency is still low and highly genotype-dependent when compared to other species. Here, we characterized seventeen soybean genotypes based on their genetic transformation efficiencies, i.e., high and low, during Agrobacterium-mediated transformation. To reveal the molecular basis of this transformation difference, we constructed a highly efficient transient transgene expression system using soybean cotyledon protoplasts and then assess the methylation levels of promoter and coding regions of an EYFP (enhanced yellow fluorescent protein) gene introduced into the protoplast cultures of various soybean genotypes using BSP (bisulfite sequencing PCR). Increased methylation was found to be associated with the considerably decreased transfection efficiency (as percentage of EYFP fluorescent protoplasts) in low-efficacy genotypes as compared with those in high-efficacy on three DAT (day after transfection). 5-Azacytidine (5-Azac), a demethylating reagent commonly applied in epigenetic researches, significantly improved the transient transfection efficiency and transgene expression level in low-efficiency genotypes. Furthermore, the shoot regeneration efficiency in low-efficiency genotypes was substantially increased by 5-Azac treatment in an Agrobacterium-mediated soybean transformation system. Taken together, we concluded that lower methylation level in transgene contributed to enhanced shoot regeneration in Agrobacterium-mediated soybean transformation.

Development and Characterization of a Novel, High-Affinity, Specific, Radiolabeled Ligand for BRS-3 Receptors

Bombesin (Bn) receptor subtype 3(BRS-3) is an orphan G-protein-coupled receptor of the Bn family, which does not bind any natural Bn peptide with high affinity. Receptor knockout studies show that the animals develop diabetes, obesity, altered temperature control, and other central nervous system (CNS)/endocrine/gastrointestinal changes. It is present in CNS, peripheral tissues, and tumors; however, its role in normal physiology/pathophysiology, as well as its receptor localization/pharmacology is largely unknown, in part due to the lack of a convenient, specific, direct radiolabeled ligand. This study was designed to address this problem and to develop and characterize a specific radiolabeled ligand for BRS-3. The peptide antagonist Bantag-1 had >10,000-fold selectivity for human BRS-3 (hBRS-3) over other mammalian Bn receptors (BnRs) [i.e., gastrin-releasing peptide receptor (GRPR) and neuromedin B receptor (NMBR)]. Using iodogen and basic conditions, it was radiolabeled to high specific activity (2200 Ci/mmol) and found to bind with high affinity/specificity to hBRS-3. Binding was saturable, rapid, and reversible. The ligand only interacted with known BRS-3 ligands, and not with other specific GRPR/NMBR ligands or ligands for unrelated receptors. The magnitude of 125I-Bantag-1 binding correlated with BRS-3 mRNA expression and the magnitude of activation of phospholipase C in lung cancer cells, as well as readily identifying BRS-3 in lung cancer cells and normal tissues, allowing the direct assessment of BRS-3 receptor pharmacology/numbers on cells containing BRS-3 with other BnRs, which is usually the case. This circumvents the need for subtraction assays, which are now frequently used to assess BRS-3 indirectly using radiolabeled pan-ligands, which interact with all BnRs.

A possible new target in lung-cancer cells: The orphan receptor, bombesin receptor subtype-3

Human bombesin receptors, GRPR and NMBR, are two of the most frequently overexpressed G-protein-coupled-receptors by lung-cancers. Recently, GRPR/NMBR are receiving considerable attention because they act as growth factor receptors often in an autocrine manner in different lung-cancers, affect tumor angiogenesis, their inhibition increases the cytotoxic potency of tyrosine-kinase inhibitors reducing lung-cancer cellular resistance/survival and their overexpression can be used for sensitive tumor localization as well as to target cytotoxic agents to the cancer. The orphan BRS-3-receptor, because of homology is classified as a bombesin receptor but has received little attention, despite the fact that it is also reported in a number of studies in lung-cancer cells and has growth effects in these cells. To address its potential importance, in this study, we examined the frequency/relative quantitative expression of human BRS-3 compared to GRPR/NMBR and the effects of its activation on cell-signaling/growth in 13 different human lung-cancer cell-lines. Our results showed that BRS-3 receptor is expressed in 92% of the cell-lines and that it is functional in these cells, because its activation stimulates phospholipase-C with breakdown of phosphoinositides and changes in cytosolic calcium, stimulates ERK/MAPK and stimulates cell growth by EGFR transactivation in some, but not all, the lung-cancer cell-lines. These results suggest that human BRS-3, similar to GRPR/NMBR, is frequently ectopically-expressed by lung-cancer cells in which, it is functional, affecting cell signaling/growth. These results suggest that similar to GRPR/NMBR, BRS-3 should receive increased attention as possible approach for the development of novel treatments and/or diagnosis in lung-cancer.

A Selective Bombesin Receptor Subtype 3 Agonist Promotes Weight Loss in Male Diet-Induced-Obese Rats With Circadian Rhythm Change

Bombesin receptor subtype 3 (BRS-3) is an orphan G protein-coupled receptor. Based on the obese phenotype of male BRS-3-deficient mice, BRS-3 has been considered an attractive target for obesity treatment. Here, we developed a selective BRS-3 agonist (compound-A) and evaluated its antiobesity effects. Compound-A showed anorectic effects and enhanced energy expenditure in diet-induced-obese (DIO)-F344 rats. Moreover, repeated oral administration of compound-A for 7 days resulted in a significant body weight reduction in DIO-F344 rats. We also evaluated compound-A for cardiovascular side effects using telemeterized Sprague-Dawley (SD) rats. Oral administration of compound-A resulted in transient blood pressure increases in SD rats. To investigate the underlying mechanisms of BRS-3 agonist effects, we focused on the suprachiasmatic nucleus (SCN), the main control center of circadian rhythms in the hypothalamus, also regulating sympathetic nervous system. Compound-A significantly increased the messenger RNA expression of Brs-3, c-fos, and circadian rhythm genes in SCN of DIO-F344 rats. Because SCN also controls the hypothalamic-pituitary-adrenal (HPA) axis, we evaluated the relationship between BRS-3 and the HPA axis. Oral administration of compound-A caused a significant increase of plasma corticosterone levels in DIO-F344 rats. On this basis, energy expenditure enhancement by compound-A may be due to a circadian rhythm change in central and peripheral tissues, enhancement of peripheral lipid metabolism, and stimulation of the sympathetic nervous system. Furthermore, the blood pressure increase by compound-A could be associated with sympathetic nervous system stimulation via SCN and elevation of plasma corticosterone levels through activation of the HPA axis.

Molecular basis for high affinity and selectivity of peptide antagonist, Bantag-1, for the orphan BB3 receptor

Bombesin-receptor-subtype-3 (BB3 receptor) is a G-protein-coupled-orphan-receptor classified in the mammalian Bombesin-family because of high homology to gastrin-releasing peptide (BB2 receptor)/neuromedin-B receptors (BB1 receptor). There is increased interest in BB3 receptor because studies primarily from knockout-mice suggest it plays roles in energy/glucose metabolism, insulin-secretion, as well as motility and tumor-growth. Investigations into its roles in physiological/pathophysiological processes are limited because of lack of selective ligands. Recently, a selective, peptide-antagonist, Bantag-1, was described. However, because BB3 receptor has low-affinity for all natural, Bn-related peptides, little is known of the molecular basis of its high-affinity/selectivity. This was systematically investigated in this study for Bantag-1 using a chimeric-approach making both Bantag-1 loss-/gain-of-affinity-chimeras, by exchanging extracellular (EC) domains of BB3/BB2 receptor, and using site-directed-mutagenesis. Receptors were transiently expressed and affinities determined by binding studies. Bantag-1 had >5000-fold selectivity for BB3 receptor over BB2/BB1 receptors and substitution of the first EC-domain (EC1) in loss-/gain-of affinity-chimeras greatly affected affinity. Mutagenesis of each amino acid difference in EC1 between BB3 receptor/BB2 receptor showed replacement of His(107) in BB3 receptor by Lys(107) (H107K-BB3 receptor-mutant) from BB2 receptor, decreased affinity 60-fold, and three replacements [H107K, E11D, G112R] decreased affinity 500-fold. Mutagenesis in EC1's surrounding transmembrane-regions (TMs) demonstrated TM2 differences were not important, but R127Q in TM3 alone decreased affinity 400-fold. Additional mutants in EC1/TM3 explored the molecular basis for these changes demonstrated in EC1, particularly important is the presence of aromatic-interactions by His(107), rather than hydrogen-bonding or charge-charge interactions, for determining Bantag-1 high affinity/selectivity. In regard to Arg(127) in TM3, both hydrogen-bonding and charge-charge interactions contribute to the high-affinity/selectivity for Bantag-1.

Novel chiral-diazepines function as specific, selective receptor agonists with variable coupling and species variability in human, mouse and rat BRS-3 receptor cells

Bombesin receptor subtype-3 (BRS-3) is an orphan G-protein coupled receptor which is classified in the bombesin receptor (BnR) family with which it shares high homology. It is present widely in the central nervous system and peripheral tissues and primarily receptor-knockout studies suggest it is involved in metabolic-glucose-insulin homeostasis, feeding and other CNS behaviors, gastrointestinal motility and cancer growth. However, the role of BRS-3 physiologically or in pathologic disorders has been not well defined because the natural ligand is unknown. Until recently, no selective agonists/antagonists were available; however, recently synthetic high-affinity agonists, chiral-diazepines nonpeptide-analogs (3F, 9D, 9F, 9G) with low CNS penetrance, were described, but are not well-categorized pharmacologically or in different labarotory species. The present study characterizes the affinities, potencies, selectivities of the chiral-diazepine BRS-3 agonists in human and rodents (mice,rat). In human BRS-3 receptors, the relative affinities of the chiral-diazepines was 9G>9D>9F>3F; each was selective for BRS-3. For stimulating PLC activity, in h-BRS-3 each of the four chiral diazepine analogs was fully efficacious and their relative potencies were: 9G (EC50: 9 nM)>9D (EC50: 9.4 nM)>9F (EC50: 39 nM)>3F (EC50: 48 nM). None of the four chiral diazepine analogs activated r,m,h-GRPR/NMBR. The nonpeptide agonists showed marked differences from each other and a peptide agonist in receptor-coupling-stiochiometry and in affinities/potencies in different species. These results demonstrate that chiral diazepine analogs (9G, 9D, 9F, 3F) have high/affinity/potency for the BRS-3 receptor in human and rodent cells, but different coupling-relationships and species differences from a peptide agonist.

Insights into bombesin receptors and ligands: Highlighting recent advances

This following article is written for Prof. Abba Kastin's Festschrift, to add to the tribute to his important role in the advancement of the role of peptides in physiological, as well as pathophysiological processes. There have been many advances during the 35 years of his prominent role in the Peptide field, not only as editor of the journal Peptides, but also as a scientific investigator and editor of two volumes of the Handbook of Biological Active Peptides [146,147]. Similar to the advances with many different peptides, during this 35 year period, there have been much progress made in the understanding of the pharmacology, cell biology and the role of (bombesin) Bn receptors and their ligands in various disease states, since the original isolation of bombesin from skin of the European frog Bombina bombina in 1970 [76]. This paper will briefly review some of these advances over the time period of Prof. Kastin 35 years in the peptide field concentrating on the advances since 2007 when many of the results from earlier studies were summarized [128,129]. It is appropriate to do this because there have been 280 articles published in Peptides during this time on bombesin-related peptides and it accounts for almost 5% of all publications. Furthermore, 22 Bn publications we have been involved in have been published in either Peptides [14,39,55,58,81,92,93,119,152,216,225,226,231,280,302,309,355,361,362] or in Prof. Kastin's Handbook of Biological Active Peptides [137,138,331].

Bombesin receptor subtype 3 as a potential target for obesity and diabetes

INTRODUCTION

Diabetes mellitus and obesity are important health issues; increasing in prevalence, both in the USA and globally. There are only limited pharmacological treatments, and although bariatric surgery is effective, new effective pharmacologic treatments would be of great value. This review covers one area of increasing interest that could yield new novel treatments of obesity/diabetes mellitus. It involves recognition of the central role the G-protein-coupled receptor, bombesin receptor subtype 3 (BRS-3) plays in energy/glucose metabolism.

AREAS COVERED

Since the initial observation that BRS-3 knockout mice develop obesity, hypertension, impaired glucose metabolism and hyperphagia, there have been numerous studies of the mechanisms involved and the development of selective BRS-3 agonists/antagonists, which have marked effects on body weight, feeding and glucose/insulin homeostasis. In this review, each of these areas is briefly reviewed.

EXPERT OPINION

BRS-3 plays an important role in glucose/energy homeostasis. The development of potent, selective BRS-3 agonists demonstrates promise as a novel approach to treat obesity/diabetic states. One important question that needs to be addressed is whether BRS-3 agonists need to be centrally acting. This is particularly important in light of recent animal and human studies that report transient cardiovascular side effects with centrally acting oral BRS agonists.

The Nutrient-Responsive Hormone CCHamide-2 Controls Growth by Regulating Insulin-like Peptides in the Brain of Drosophila melanogaster

The coordination of growth with nutritional status is essential for proper development and physiology. Nutritional information is mostly perceived by peripheral organs before being relayed to the brain, which modulates physiological responses. Hormonal signaling ensures this organ-to-organ communication, and the failure of endocrine regulation in humans can cause diseases including obesity and diabetes. In Drosophila melanogaster, the fat body (adipose tissue) has been suggested to play an important role in coupling growth with nutritional status. Here, we show that the peripheral tissue-derived peptide hormone CCHamide-2 (CCHa2) acts as a nutrient-dependent regulator of Drosophila insulin-like peptides (Dilps). A BAC-based transgenic reporter revealed strong expression of CCHa2 receptor (CCHa2-R) in insulin-producing cells (IPCs) in the brain. Calcium imaging of brain explants and IPC-specific CCHa2-R knockdown demonstrated that peripheral-tissue derived CCHa2 directly activates IPCs. Interestingly, genetic disruption of either CCHa2 or CCHa2-R caused almost identical defects in larval growth and developmental timing. Consistent with these phenotypes, the expression of dilp5, and the release of both Dilp2 and Dilp5, were severely reduced. Furthermore, transcription of CCHa2 is altered in response to nutritional levels, particularly of glucose. These findings demonstrate that CCHa2 and CCHa2-R form a direct link between peripheral tissues and the brain, and that this pathway is essential for the coordination of systemic growth with nutritional availability. A mammalian homologue of CCHa2-R, Bombesin receptor subtype-3 (Brs3), is an orphan receptor that is expressed in the islet β-cells; however, the role of Brs3 in insulin regulation remains elusive. Our genetic approach in Drosophila melanogaster provides the first evidence, to our knowledge, that bombesin receptor signaling with its endogenous ligand promotes insulin production.

Animals need to couple growth with nutritional availability for proper development and physiology, which leads to better survival. Nutritional information is mostly perceived by peripheral organs, particularly metabolic organs such as adipose tissue and gut, before being relayed to the brain, which modulates physiological responses. Hormonal signaling ensures this organ-to-organ communication, and defects in this endocrine regulation in humans often cause diseases including obesity and diabetes. In the fruit fly Drosophila melanogaster, adipose tissue (the “fat body”) has been suggested to play an important role in coordinating growth with metabolism. Here, we show that the Drosophila CCHamide-2 (CCHa2) gene, expressed in the fat body and gut, encodes a nutrient-sensitive peptide hormone. The CCHa2 peptide signals to neuroendocrine cells in the brain that produce Drosophila insulin-like peptides (Dilps) through its receptor (CCHa2-R) and promotes the production of Dilps. Mutants of both CCHa2 and CCHa2-R display severe growth retardation during larval stages. These results suggest that CCHa2 and CCHa2-R functionally connect peripheral tissues with the brain, and that CCHa2/CCHa2-R signaling coordinates the animal’s growth with its nutritional conditions by regulating its production of insulin-like peptides.

Comparative pharmacology of bombesin receptor subtype-3, nonpeptide agonist MK-5046, a universal peptide agonist, and peptide antagonist Bantag-1 for human bombesin receptors

Bombesin-receptor-subtype-3 (BRS-3) is an orphan G-protein-coupled receptor of the bombesin (Bn) family whose natural ligand is unknown and which does not bind any natural Bn-peptide with high affinity. It is present in the central nervous system, peripheral tissues, and tumors; however, its role in normal physiology/pathophysiology is largely unknown because of the lack of selective ligands. Recently, MK-5046 [(2S)-1,1,1-trifluoro-2-[4-(1H-pyrazol-1-yl)phenyl]-3-(4-{[1-(trifluoromethyl)cyclopropyl]methyl}-1H-imidazol-2-yl)propan-2-ol] and Bantag-1 [Boc-Phe-His-4-amino-5-cyclohexyl-2,4,5-trideoxypentonyl-Leu-(3-dimethylamino) benzylamide N-methylammonium trifluoroacetate], a nonpeptide agonist and a peptide antagonist, respectively, for BRS-3 have been described, but there have been limited studies on their pharmacology. We studied MK-5046 and Bantag-1 interactions with human Bn-receptors-human bombesin receptor subtype-3 (hBRS-3), gastrin-releasing peptide receptor (GRP-R), and neuromedin B receptor (NMB-R)-and compared them with the nonselective, peptide-agonist [d-Tyr6,βAla11,Phe13,Nle14]Bn-(6-14) (peptide #1). Receptor activation was detected by activation of phospholipase C (PLC), mitogen-activated protein kinase (MAPK), focal adhesion kinase (FAK), paxillin, and Akt. In hBRS-3 cells, the relative affinities were Bantag-1 (1.3 nM) > peptide #1 (2 nM) > MK-5046 (37-160 nM) > GRP, NMB (>10 μM), and the binding-dose-inhibition curves were broad (>4 logs), with Hill coefficients differing significantly from unity. Curve-fitting demonstrated high-affinity (MK-5046, Ki = 0.08 nM) and low-affinity (MK-5046, Ki = 11-29 nM) binding sites. For PLC activation in hBRS-3 cells, the relative potencies were MK-5046 (0.02 nM) > peptide #1 (6 nM) > GRP, NMB, Bantag-1 (>10 μM), and MK-5046 had a biphasic dose response, whereas peptide #1 was monophasic. Bantag-1 was a specific hBRS-3-antagonist. In hBRS-3 cells, MK-5046 was a full agonist for activation of MAPK, FAK, Akt, and paxillin; however, it was a partial agonist for phospholipase A2 (PLA2) activation. The kinetics of activation/duration of action for PLC/MAPK activation of MK-5046 and peptide #1 differed, with peptide #1 causing more rapid stimulation; however, MK-5046 had more prolonged activity. Our study finds that MK-5046 and Bantag-1 have high affinity/selectivity for hBRS-3. The nonpeptide MK-5046 and peptide #1 agonists differ markedly in their receptor coupling, ability to activate different signaling cascades, and kinetics/duration of action. These results show that their hBRS-3 receptor activation is not always concordant and could lead to markedly different cellular responses.

Bombesin receptor subtype-3 (BRS-3), a novel candidate as therapeutic molecular target in obesity and diabetes

BRS-3 KO-mice developed obesity and unbalanced glucose metabolism, suggesting an important role of BRS-3 receptor in glucose homeostasis. We explored BRS-3 expression in skeletal muscle from normal, obese or type-2 diabetic (T2D) patients, and the effect of [D-Phe(6), β-Ala(11),Phe(13),Nle(14)]bombesin(6-14)-BRS-3-agonist-peptide (BRS-3-AP) - on glucose-related effects, before or after BRS-3 gene silencing. In muscle tissue and primary cultured myocytes from altered metabolic states, BRS-3 gene/protein expressions were down-regulated. In normal, obese and T2D cells: A) BRS-3-AP as insulin enhanced BRS-3 and GLUT-4 mRNA/protein levels; improving glucotransporter translocation to plasma membrane, and B) BRS-3-AP caused a concentration-related-stimulation of glucose transport, being obese and T2D myocytes more sensitive to the ligand than normal. Wortmannin and PD98059, but not rapamycin, abolished the stimulatory action of BRS-3-AP on glucose transport. BRS-3 plays an important role in glucose metabolism, and could be use as a molecular target, and/or its ligand, as a therapeutic agent for obesity and diabetes treatments.

(68)Ga-labeled bombesin analogs for receptor-mediated imaging

Targeted receptor-mediated imaging techniques have become crucial tools in present targeted diagnosis and radiotherapy as they provide accurate and specific diagnosis of disease information. Peptide-based pharmaceuticals are gaining popularity, and there has been vast interest in developing (68)Ga-labeled bombesin (Bn) analogs. The gastrin-releasing peptide (GRP) family and its Bn analog have been implicated in the biology of several human cancers. The three bombesin receptors GRP, NMB, and BRS-3 receptor are most frequently ectopically expressed by common, important malignancies. The low expression of Bn/GRP receptors in normal tissue and relatively high expression in a variety of human tumors can be of biological importance and form a molecular basis for Bn/GRP receptor-mediated imaging. To develop a Bn-like peptide with favorable tumor targeting and pharmacokinetic characteristics for possible clinical use, several modifications in the Bn-like peptides, such as the use of a variety of chelating agents, i.e., acyclic and macrocyclic agents with different spacer groups and with different metal ions (gallium), have been performed in recent years without significant disturbance of the vital binding scaffold. The favorable physical properties of (68)Ga, i.e., short half-life, and the fast localization of small peptides make this an ideal combination to study receptor-mediated imaging in patients.

The molecular basis for high affinity of a universal ligand for human bombesin receptor (BnR) family members

There is increased interest in the Bn-receptor family because they are frequently over/ectopically expressed by tumors and thus useful as targets for imaging or receptor-targeted-cytotoxicity. The synthetic Bn-analog, [D-Tyr(6), β-Ala(11), Phe(13), Nle(14)]Bn(6-14) [Univ.Lig] has the unique property of having high affinity for all three human BNRs (GRPR, NMBR, BRS-3), and thus could be especially useful for this approach. However, the molecular basis of this property is unclear and is the subject of this study. To accomplish this, site-directed mutagenesis was used after identifying potentially important amino acids using sequence homology analysis of all BnRs with high affinity for Univ.Lig compared to the Cholecystokinin-receptor (CCK(A)R), which has low affinity. Using various criteria 74 amino acids were identified and 101 mutations made in GRPR by changing each to those of CCK(A)R or to alanine. 22 GRPR mutations showed a significant decrease in affinity for Univ.Lig (>2-fold) with 2 in EC2[D97N, G112V], 1 in UTM6[Y284A], 2 in EC4[R287N, H300S] showing >10-fold decrease in Univ.Lig affinity. Additional mutations were made to explore the molecular basis for these changes. Our results show that high affinity for Univ.Lig by human Bn-receptors requires positively charged amino acids in extracellular (EC)-domain 4 and to a lesser extent EC2 and EC3 suggesting charge-charge interactions may be particularly important for determining the general high affinity of this ligand. Furthermore, transmembrane amino acids particularly in UTM6 are important contributing both charge-charge interactions as well as interaction with a tyrosine residue in close proximity suggesting possible receptor-peptide cation-π or H-bonding interactions are also important for determining its high affinity.

Biology and pharmacology of bombesin receptor subtype-3

PURPOSE OF REVIEW

This review summarizes the results of recent studies regarding the biology and pharmacology of novel synthetic agonists and antagonists of the bombesin receptor subtype-3 (BRS-3).

RECENT FINDINGS

All three mammalian bombesin receptors including gastrin-releasing peptide receptor, the neuromedin B receptor, and the BRS-3 have been shown to regulate energy balance and appetite and satiety. Studies indicate that the orphan BRS-3 is an important regulator of body weight, energy expenditure, and glucose homeostasis. Endogenous bombesin-like peptides bombesin, gastrin-releasing peptide, and neuromedin B receptor do not bind to BRS-3 and the endogenous BRS-3 ligand remains unknown. The novel synthesis of selective, high-affinity BRS-3 agonists and antagonists has recently been accomplished and showed that BRS-3 regulates energy balance independent of other established pathways and glucose-stimulated insulin secretion in the pancreatic islet cells. The availability of new BRS-3 selective agonists and antagonists will facilitate further elucidation of its role in energy homeostasis, and provides a potential approach for the pharmacological treatment of obesity and type 2 diabetes.

SUMMARY

The native ligand of the G protein-coupled BRS-3 has not been identified as of now. However, novel synthesis of small-molecule, high-affinity agonists and antagonists on the BRS-3 was used in the recent studies and demonstrated an important role of BRS-3 in the regulation of energy homeostasis and glucose metabolism.

Appetite-modifying effects of bombesin receptor subtype-3 agonists

Studies on bombesin-like peptides (BLP) and their respective mammalian receptors (Bn-r) have demonstrated a significant biological impact on a broad array of physiological and pathophysiological conditions. Pharmacological experiments in vitro and in vivo as well as utilization of genetic rodent models of the gastrin-releasing peptide receptor (GRP-R/BB2-receptor), neuromedin B receptor (NMB-R/BB1-receptor), and the bombesin receptor subtype-3 (BRS-3/BB3-receptor) further delineated their role in health and disease. All three mammalian bombesin receptors have been shown to possess some role in the regulation of energy balance and appetite and satiety. Compelling experimental evidence has accumulated indicating that the orphan BRS-3 is an important regulator of body weight, energy expenditure, and glucose homeostasis. BRS-3 possesses no high affinity to the endogenous bombesin-like peptides (BLP) bombesin, GRP, and NMB, and its endogenous ligand remains unknown. Recently, the synthesis of novel, selective high-affinity BRS-3 agonists and antagonists has been accomplished and has demonstrated that BRS-3 regulates energy balance independent of other established pathways. Accordingly, the availability of new BRS-3 selective agonists and antagonists will facilitate further elucidation of its role in energy homeostasis and provides a potential approach for the pharmacological treatment of obesity.

Pharmacology and selectivity of various natural and synthetic bombesin related peptide agonists for human and rat bombesin receptors differs

The mammalian bombesin (Bn)-receptor family [gastrin-releasing peptide-receptor (GRPR-receptor), neuromedin B-receptor (NMB receptor)], their natural ligands, GRP/NMB, as well as the related orphan receptor, BRS-3, are widely distributed, and frequently overexpressed by tumors. There is increased interest in agonists for this receptor family to explore their roles in physiological/pathophysiological processes, and for receptor-imaging/cytotoxicity in tumors. However, there is minimal data on human pharmacology of Bn receptor agonists and most results are based on nonhuman receptor studies, particular rodent-receptors, which with other receptors frequently differ from human-receptors. To address this issue we compared hNMB-/GRP-receptor affinities and potencies/efficacies of cell activation (assessing phospholipase C activity) for 24 putative Bn-agonists (12 natural, 12 synthetic) in four different cells with these receptors, containing native receptors or receptors expressed at physiological densities, and compared the results to native rat GRP-receptor containing cells (AR42J-cells) or rat NMB receptor cells (C6-glioblastoma cells). There were close correlations (r=0.92-99, p<0.0001) between their affinities/potencies for the two hGRP- or hNMB-receptor cells. Twelve analogs had high affinities (≤ 1 nM) for hGRP receptor with 15 selective for it (greatest=GRP, NMC), eight had high affinity/potencies for hNMB receptors and four were selective for it. Only synthetic Bn analogs containing β-alanine(11) had high affinity for hBRS-3, but also had high affinities/potencies for all GRP-/hNMB-receptor cells. There was no correlation between affinities for human GRP receptors and rat GRP receptors (r=0.131, p=0.54), but hNMB receptor results correlated with rat NMB receptor (r=0.71, p<0.0001). These results elucidate the human and rat GRP-receptor pharmacophore for agonists differs markedly, whereas they do not for NMB receptors, therefore potential GRP-receptor agonists for human studies (such as Bn receptor-imaging/cytotoxicity) must be assessed on human Bn receptors. The current study provides affinities/potencies on a large number of potential agonists that might be useful for human studies.

Pharmacology of putative selective hBRS-3 receptor agonists for human bombesin receptors (BnR): affinities, potencies and selectivity in multiple native and BnR transfected cells

The orphan receptor, bombesin receptor subtype-3(BRS-3) is a G-protein-coupled receptor classified in the bombesin (Bn) receptor family because of its high homology (47-51%) with other members of this family [gastrin-releasing peptide receptor [GRPR] and neuromedin B receptor [NMBR]]. There is increasing interest in BRS-3, because primarily from receptor knockout studies, it seems important in energy metabolism, glucose control, insulin secretion, motility and tumor growth. Pharmacological tools to study the role of BRS-3 in physiology/pathophysiology are limited because the natural ligand is unknown and BRS-3 has low affinity for all naturally occurring Bn-related peptides. However, a few years ago a synthetic high-affinity agonist [dTyr(6),betaAla(11),Phe(13),Nle(14)]Bn-(6-14) was described but was nonselective for BRS-3 over other Bn receptors. Based on this peptide, in various studies a number of putative selective, high-potency hBRS-3 agonists were described, however the results on their selectivity are conflicting in a number of cases. The purpose of the present study was to thoroughly study the pharmacology of four of the most select/potent putative hBRS-3 agonists (#2-4, 16a). Each was studied in multiple well-characterized Bn receptor-transfected cells and native Bn receptor bearing cells, using binding studies, alterations in cellular signaling (PLC, PKD) and changes in cellular function(growth). Two peptides (#2, #3) had nM affinities/potencies for hBRS-3, peptide #4 had low affinity/potency, and peptide #16a very low (>3000 nM). Peptide#3 had the highest selectivity for hBRS-3 (100-fold), whereas #2, 4 had lower selectivity. Peptide #16a's selectivity could not be determined because of its low affinity/potencies for all hBn receptors. These results show that peptide #3 is the preferred hBRS-3 agonist for studies at present, although its selectivity of only 100-fold may limit its utility in some cases. This study underscores the importance of full pharmacological characterization of newly reported selective agonists.

Molecular basis for agonism in the BB3 receptor: an epitope located on the interface of transmembrane-III, -VI, and -VII

Epitopes determining the agonist property of two structurally distinct selective ligands for the human bombesin receptor subtype 3 (BB3), [D-Tyr6,(R)-Apa11,Phe13, Nle14]-bombesin(6-14) (Pep-1) and Ac-Phe-Trp-Ala-His(TauBzl)-Nip-Gly-Arg-NH2 (Pep-2), were mapped through systematic mutagenesis of the main ligand-binding pocket of the receptor. The mutational map for the smaller Pep-2 spanned the entire binding pocket of the BB3 receptor. In contrast, the much fewer mutational hits for the larger Pep-1 were confined to the center of the pocket, i.e., the opposing faces of the extracellular segments of transmembrane (TM)-III, TM-VI, and TM-VII. All the residues, which upon mutation affected Pep-1, were also hits for Pep-2 and included those that were most essential for the function of Pep-2: LeuIII:04 (Leu(123)), TyrVI:16 (Tyr(291)), and ArgVII:06 (Arg(316)). The BB3 receptor was found to signal with 12% ligand-independent activity that was strongly influenced both positively and negatively by several mutations in the binding pocket. The substitutions, which decreased the constitutive signaling, included not only the major mutational hits for the peptide agonists but also mutations more superficially located in the receptor. It is concluded that activation of the BB3 receptor is dependent upon an epitope in the main ligand-binding pocket at the interface between TM-III, TM-VI, and TM-VII that corresponds to the site where, for example, activating metal ion sites have been constructed previously in 7TM receptors.

Characterization of putative GRP- and NMB-receptor antagonist's interaction with human receptors

The mammalian bombesin (Bn) peptides neuromedin B (NMB) and gastrin-releasing peptide (GRP) actions are mediated by two receptors (NMB-receptor, GRP-receptor) which are widely distributed in the GI tract and CNS. From primarily animal studies NMB/GRP-receptor activation has physiological/pathophysiological effects in the CNS and GI tract including stimulating of growth of cancers and normal tissues. Whereas these Bn-receptors' effects have been extensively studied in nonhuman cells and animals, little is known of the physiological/pathological role(s) in humans, largely due to lack of potent antagonists. To address this issue we compared NMB/GRP-receptor affinity/potency of 10 chemical classes of putative antagonists (35 compounds) for human Bn-receptors by performing binding studies or assessing abilities to activate hGRP/hNMB-receptor [assessing phospholipase C activation] in four different cells containing native Bn-receptors or transfected receptors. From binding studies 23 were GRP-receptor-preferring, 4 were NMB-receptor, and 8 nonselective. For the hGRP-receptor-preferring analogues none showed hGRP-receptor agonist activity, but 13 were full or partial hNMB-receptor agonists at hNMB-receptors. For hNMB-receptor-preferring analogues none were agonists. Analogue #24 ([(3-Ph-Pr(6)), His(7), d-Ala(11), d-Pro(13), Psi(13-14), Phe(14)]Bn(6-14)NH2) and analogue #7 [d-Phe(6), Leu(13), Psi(CH(2)NH), Cpa(14)]Bn(6-14) were the most potent (0.2-1.4nM) and selective (>10,000-fold) for the hGRP-receptor with analogue #7.5 [d-Tpi(6), Leu(13), Psi(CH2NH), Leu(14)]Bn(6-14)[RC-3095] (0.2-1.4nM) slightly less selective. Analogue #34 (PD168368) had the highest affinity for hNMB-receptor (1.32-1.58nM) and the greatest selectivity (2298-6952-fold) for the hNMB-receptor. These results demonstrate numerous putative hGRP/hNMB-receptor antagonists identified in nonhuman cells and/or animals have agonist activity at the hNMB-receptor, limiting their potential usefulness. However, a number were identified which were potent/selective for human Bn-receptors and should be useful for investigating their roles in human physiological/pathophysiological conditions.

Bombesin-modified 6-14 C-terminal fragments adsorption on silver surfaces: influence of a surface substrate

Surface-enhanced Raman scattering (SERS) spectroscopy has been applied to investigate the interaction with a silver colloidal surface of following seven 6-14 fragments of bombesin (BN) C-terminus: cyclo[D-Phe(6),His(7),Leu(14)]BN(6-14), [D-Phe(6),Leu-NHEt(13),des-Met(14)]BN(6-14), [D-Phe(6),Leu(13)-(R)-p-chloro-Phe(14)]BN(6-14), [D-Phe(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), [D-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), [D-Tyr(6),beta-Phe(11),Phe(13),Nle(14)OH]BN(6-14), and [D-Cys(6),Asn(7),D-Ala(11),Cys(14)]BN(6-14), potent r-GRP-R receptor antagonists used in chemotherapy and potential effective drugs in cancer treatment. The adsorption active sites and molecular orientations on the colloidal silver surface have been determined on the basis of SERS "surface selection rules" subsequent to a detailed SERS analysis. In addition, the similarities and differences of these spectra with the SERS spectra of the peptides immobilized on a roughened silver electrode surface have been examined. From the data, suggestion has been made about structural properties of these peptides on the colloidal surface.

Gastrin-releasing peptide (GRP) in the ovine uterus: regulation by interferon tau and progesterone

Gastrin-releasing peptide (GRP) is abundantly expressed by endometrial glands of the ovine uterus and processed into different bioactive peptides, including GRP1-27, GRP18-27, and a C-terminus, that affect cell proliferation and migration. However, little information is available concerning the hormonal regulation of endometrial GRP and expression of GRP receptors in the ovine endometrium and conceptus. These studies determined the effects of pregnancy, progesterone (P4), interferon tau (IFNT), placental lactogen (CSH1), and growth hormone (GH) on expression of GRP in the endometrium and GRP receptors (GRPR, NMBR, BRS3) in the endometrium, conceptus, and placenta. In pregnant ewes, GRP mRNA and protein were first detected predominantly in endometrial glands after Day 10 and were abundant from Days 18 through 120 of gestation. Treatment with IFNT and progesterone but not CSH1 or GH stimulated GRP expression in the endometrial glands. Western blot analyses identified proGRP in uterine luminal fluid and allantoic fluid from Day 80 unilateral pregnant ewes but not in uterine luminal fluid of either cyclic or early pregnant ewes. GRPR mRNA was very low in the Day 18 conceptus and undetectable in the endometrium and placenta; NMBR and BRS3 mRNAs were undetectable in ovine uteroplacental tissues. Collectively, the present studies validate GRP as a novel IFNT-stimulated gene in the glands of the ovine uterus, revealed that IFNT induction of GRP is dependent on P4, and found that exposure of the ovine uterus to P4 for 20 days induces GRP expression in endometrial glands.

International Union of Pharmacology. LXVIII. Mammalian bombesin receptors: nomenclature, distribution, pharmacology, signaling, and functions in normal and disease states

The mammalian bombesin receptor family comprises three G protein-coupled heptahelical receptors: the neuromedin B (NMB) receptor (BB(1)), the gastrin-releasing peptide (GRP) receptor (BB(2)), and the orphan receptor bombesin receptor subtype 3 (BRS-3) (BB(3)). Each receptor is widely distributed, especially in the gastrointestinal (GI) tract and central nervous system (CNS), and the receptors have a large range of effects in both normal physiology and pathophysiological conditions. The mammalian bombesin peptides, GRP and NMB, demonstrate a broad spectrum of pharmacological/biological responses. GRP stimulates smooth muscle contraction and GI motility, release of numerous GI hormones/neurotransmitters, and secretion and/or hormone release from the pancreas, stomach, colon, and numerous endocrine organs and has potent effects on immune cells, potent growth effects on both normal tissues and tumors, potent CNS effects, including regulation of circadian rhythm, thermoregulation; anxiety/fear responses, food intake, and numerous CNS effects on the GI tract as well as the spinal transmission of chronic pruritus. NMB causes contraction of smooth muscle, has growth effects in various tissues, has CNS effects, including effects on feeding and thermoregulation, regulates thyroid-stimulating hormone release, stimulates various CNS neurons, has behavioral effects, and has effects on spinal sensory transmission. GRP, and to a lesser extent NMB, affects growth and/or differentiation of various human tumors, including colon, prostate, lung, and some gynecologic cancers. Knockout studies show that BB(3) has important effects in energy balance, glucose homeostasis, control of body weight, lung development and response to injury, tumor growth, and perhaps GI motility. This review summarizes advances in our understanding of the biology/pharmacology of these receptors, including their classification, structure, pharmacology, physiology, and role in pathophysiological conditions.

Molecular basis for agonist selectivity and activation of the orphan bombesin receptor subtype 3 receptor

Bombesin receptor subtype (BRS)-3, a G-protein-coupled orphan receptor, shares 51% identity with the mammalian bombesin (Bn) receptor for gastrin-releasing peptide. There is increasing interest in BRS-3 because it is important in energy metabolism, glucose control, motility, and tumor growth. BRS-3 has low affinity for all Bn-related peptides; however, recently synthetic high-affinity agonists, [d-Tyr(6)/d-Phe(6),betaAla(11),Phe(13),Nle(14)]Bn-(6-14), were described, but they are nonselective for BRS-3 over other Bn receptors. Based on these peptides, three BRS-3-selective ligands were developed: peptide 2, [d-Tyr(6)(R)-3-amino-propionic acid(11),Phe(13),Nle(14)]Bn(6-14); peptide 3, [d-Tyr(6),(R)-Apa(11),4Cl-Phe(13),Nle(14)]Bn(6-14); and peptide 4, acetyl-Phe-Trp-Ala-His-(tBzl)-piperidine-3 carboxylic acid-Gly-Arg-NH(2). Their molecular determinants of selectivity/high affinity for BRS-3 are unknown. To address this, we used a chimeric/site mutagenesis approach. Substitution of extracellular domain 2 (EC2) of BRS-3 by the comparable gastrin-releasing peptide receptor (GRPR) domain decreased 26-, 4-, and 0-fold affinity for peptides 4, 3, and 2. Substitution of EC3 decreased affinity 4-, 11-, and 0-fold affinity for peptides 2 to 4. Ten-point mutations in the EC2 and adjacent transmembrane regions (TM2) 2 and 3 of BRS-3 were made. His107 (EC2-BRS-3) for lysine (H107K) (EC2-GRPR) decreased affinity (25- and 0-fold) for peptides 4 and 1; however, it could not be activated by either peptide. Its combination with Val101 (TM2), Gly112 (EC2), and Arg127 (TM3) resulted in complete loss-of-affinity of peptide 4. Receptor-modeling showed that each of these residues face inward and are within 4 A of the binding pocket. These results demonstrate that Val101, His107, Gly112, and Arg127 in the EC2/adjacent upper TMs of BRS-3 are critical for the high BRS3 selectivity of peptide 4. His107 in EC2 is essential for BRS-3 activation, suggesting amino-aromatic ligand/receptor interactions with peptide 4 are critical for both binding and activation. Furthermore, these result demonstrate that even though these three BRS-3-selective agonists were developed from the same template peptide, [d-Phe(6),betaAla(11),Phe(13),Nle(14)]Bn-(6-14), their molecular determinants of selectivity/high affinity varied considerably.

Identification of bombesin receptor subtype-specific ligands: effect of N-methyl scanning, truncation, substitution, and evaluation of putative reported selective ligands

Mammalian bombesin (Bn) receptors include the gastrin-releasing peptide receptor, neuromedin B receptor, and bombesin receptor subtype 3 (BRS-3). These receptors are involved in a variety of physiological/pathologic processes, including thermoregulation, secretion, motility, chemotaxis, and mitogenic effects on both normal and malignant cells. Tumors frequently overexpress these receptors, and their presence is now used for imaging and receptor-mediated cytotoxicity. For these reasons, there is an increased need to develop synthetic, selective receptor subtype-specific ligands, especially agonists for these receptors. In this study, we used a number of strategies to identify useful receptor subtype-selective ligands, including synthesizing new analogs (N-methyl-substituted constrained analogs, truncations, and substitutions) in [d-Tyr(6),betaAla(11),Phe(13),Nle(14)]Bn(6-14), which has high affinity for all Bn receptors and is metabolically stable, as well as completely pharmacologically characterized analogs recently reported to be selective for these receptors in [Ca(2+)](i) assays. Affinities and potencies of each analog were determined for each human Bn receptor subtype. N-Methyl substitutions in positions 14, 12, 11, 10, 9, and 8 did not result in selective analogs, with the exception of position 11, which markedly decreased affinity/potency. N-Terminal truncations or position 12 substitutions did not increase selectivity as previously reported by others. Of the four shortened analogs of [d-Phe(6),betaAla(11),Phe(13),Nle(14)]Bn(6-14) reported to be potent selective BRS-3 ligands on [Ca(2+)](i) assays, only AcPhe,Trp,Ala,His(tauBzl),Nip,Gly,Arg-NH(2) had moderate selectivity for hBRS-3; however, it was less selective than previously reported Apa(11) analogs, demonstrating these are still the most selective BRS-3 analogs available. However, both of these analogs should be useful templates to develop more selective BRS-3 ligands.

Activation of bombesin receptor subtype-3 stimulates adhesion of lung cancer cells

Bombesin receptor subtype-3 (BRS-3) is an orphan G protein-coupled receptor having sequence homologies to gastrin-releasing peptide and neuromedin B receptors. [d-Phe6, beta-Ala11, Phe13, Nle14]bombesin(6-14) is known to act as a synthetic receptor agonist for BRS-3. To characterize BRS-3-mediated biological responses, we examined the effect of BRS-3 activation by [d-Phe6, beta-Ala11, Phe13, Nle14]Bn(6-14) on the adhesion of the small cell lung cancer NCI-N417 cells that express native BRS-3. We found that the BRS-3 agonist stimulated adhesion of NCI-N417 cells in laminin-coated culture wells. The adhesion of the cells to laminin induced by BRS-3 activation was accompanied by an increase in vinculin-like immunoreactivity and diminished in the presence of an anti-beta1 integrin antibody, suggesting that the receptor activation stimulates focal adhesion formation. We suggest that BRS-3 may be involved in invasion and metastasis of certain cancer cells, like small cell lung cancer cells, upon attachment to laminin.

Wound repair and proliferation of bronchial epithelial cells enhanced by bombesin receptor subtype 3 activation

The present study was designed to investigate the role of bombesin receptor subtype 3 (BRS-3) in airway wound repair. The results showed that: (1) There was few expression of BRS-3 mRNA in the control group. In contrast, the expression of BRS-3 mRNA was gradually increased in the early 2 days, and peaked on the fourth day, and then decreased in the ozone-stressed AHR animal. BRS-3 mRNA was distributed in the ciliated columnar epithelium, monolayer columnar epithelium cells, scattered mesenchymal cells and Type II alveolar cells; (2) The wound repair and proliferation of bronchial epithelial cells (BECs) were accelerated in a concentration-dependent manner by BRS-3 activation with P3513, which could be inhibited by PKA inhibitor H89. The study demostrated that activation of BRS-3 may play an important role in wound repair of AHR.

Developmental expression and biological activity of gastrin-releasing peptide and its receptors in the kidney

Mammalian gastrin-releasing peptide (GRP) has a widespread distribution and multiple stimulating effects on metabolism, release of regulatory peptides, gastrointestinal and pancreatic secretions, and behavior. GRP is a potent mitogen for a number of tumor types, including colon and lung. Although GRP is known to stimulate the growth of renal tumors, little is known of its synthesis, distribution, and receptors in the developing and mature kidney. Both Northern blot analysis and RT-PCR revealed the presence of GRP mRNA in ovine kidney from midgestation through to adulthood. GRP mRNA was detected in rat kidney from embryonic day 19 to postnatal day 30 by RT-PCR. Sequence-specific radioimmunoassay demonstrated the presence of substantial amounts of fully processed amidated GRP in the ovine renal cortex and medulla. The mRNA for the major receptor subtype, GRP-R, was present in fetal and adult sheep and rat kidneys. The mRNA for the low-affinity GRP receptor, bombesin receptor subtype-3 (BRS-3), was only detected in the rat kidney. In the ovine kidney, immunohistochemistry localized GRP predominantly to the thick ascending limb of the loop of Henle. mRNAs for GRP, GRP-R, and BRS-3 were detected in the human embryonic kidney cell line HEK293, and radioimmunoassay of cell extracts and conditioned media revealed the presence of proGRP but not the amidated form. However, amidated GRP did stimulate the proliferation of these cells. These studies demonstrate that the developing and mature kidney may be previously unidentified sites of autocrine or paracrine action for GRP.

Development of bombesin analogs with conformationally restricted amino acid substitutions with enhanced selectivity for the orphan receptor human bombesin receptor subtype 3

The human bombesin receptor subtype 3 (hBRS-3) orphan receptor, which has a high homology to bombesin (Bn) receptors [gastrin-releasing peptide (GRP) and neuromedin B (NMB) receptors], is widely distributed in the rat central nervous system. Its natural ligand or role in physiology is unknown due to lack of selective ligands. Its target disruption leads to obesity, diabetes, and hypertension. A synthetic high-affinity agonist, [D-Tyr6,beta-Ala11,Phe13,Nle14]Bn(6-14), has been described, but it is nonselective for hBRS-3 over other Bn receptors; however, substitution of (R)- or (S)-amino-3-phenylpropionic acid (Apa) for beta-Ala11 resulted in a modestly selective ligand. In the present study, we have attempted to develop a more selective hBRS-3 ligand by using two strategies: substitutions on phenyl ring of Apa11 and the substitution of other conformationally restricted amino acids into position 11 of [D-Tyr6,beta-Ala11,Phe13,Nle14]Bn(6-14). Fifteen analogs were synthesized and affinities were determined for hBRS-3 and Bn receptors (hGRP-R and hNMB-R). Selective analogs were tested for their ability to activate each receptor by stimulating phospholipase C. One analog, [D-Tyr6,Apa-4Cl,Phe13,Nle14]Bn(6-14), retained high affinity for the hBRS-3 (Ki=8 nM) and had enhanced selectivity (>230-fold) for hBRS-3 over hGRP-R or hNMB-R. This analog specifically interacted with hBRS-3, fully activated hBRS-3 receptors, and was a potent agonist at the hBRS-3 receptor. This enhanced selectivity should allow this analog to be useful for investigating the possible role of hBRS-3 in physiological or pathological processes.

Identification of vasoactive nonpeptidic positive and negative modulators of adrenomedullin using a neutralizing antibody-based screening strategy

Adrenomedullin (AM) is a peptide hormone implicated in blood pressure regulation and in the pathophysiology of important diseases, such as hypertension, cancer, and diabetes. However, nonpeptidic modulators of this peptide that could be used to clinically regulate its actions are not available. We present here an efficient new method to screen a large library of small molecules. This technology was applied to the identification of positive and negative modulators of AM function. A two-tier screening strategy was developed in which the first screening entails disruption of the interaction between the peptide and a neutralizing monoclonal antibody. Selected compounds were further characterized by their ability to modulate second messengers in cells containing specific AM receptors. A parallel screen against gastrin-releasing peptide selected a different subset of molecules, confirming the specificity of the screening method. Identified AM-positive regulators reduced blood pressure in vivo, whereas AM-negative regulators mediated vasoconstriction, as predicted by the vasodilatory activity of AM. Binding of the small molecules to immobilized AM was demonstrated by surface plasmon resonance assays, with K(d) values ranging from 7.76 x 10(-9) to 4.14 x 10(-6) m. Preclinical development of AM modulators may result in useful drugs for the prevention and treatment of hypertension, cancer, and diabetes.

Bombesin-like peptides and mast cell responses: relevance to bronchopulmonary dysplasia?

Bombesin-like peptides (BLPs) are elevated in newborns who later develop bronchopulmonary dysplasia (BPD). In baboon models, anti-BLP blocking antibodies abrogate BPD. We now demonstrate hyperplasia of both neuroendocrine cells and mast cells in lungs of baboons with BPD, compared with non-BPD controls or BLP antibody-treated BPD baboons. To determine whether BLPs are proinflammatory, bombesin was administered intratracheally to mice. Forty-eight hours later, we observed increased numbers of lung mast cells. We analyzed murine mast cells for BLP receptor gene expression, and identified mRNAs encoding bombesin receptor subtype 3 and neuromedin-B receptor (NMB-R), but not gastrin-releasing peptide receptor. Only NMB-R-null mice accumulated fewer lung mast cells after bombesin treatment. Bombesin, gastrin-releasing peptide, NMB, and a bombesin receptor subtype 3-specific ligand induced mast cell proliferation and chemotaxis in vitro. These observations support a role for multiple BLPs in promoting mast cell responses, suggesting a mechanistic link between BLPs and chronic inflammatory lung diseases.

Peptides with anticancer use or potential

This review is an attempt to illustrate the diversity of peptides reported for a potential or an established use in cancer therapy. With 612 references, this work aims at covering the patents and publications up to year 2000 with many inroads in years 2001-2002. The peptides are classed according to four categories of effective (or plausible) biological mechanisms of action: receptor-interacting compounds; inhibitors of protein-protein interaction; enzymes inhibitors; nucleic acid-interacting compounds. The fifth group is made of the peptides for which no mechanism of action has been found yet. Incidentally this work provides an overview of many of the modern targets of anticancer research.

Pharmacological characteristics of bombesin receptor mediating hypothermia in the central nervous system of rats

Bombesin (BN) and structurally related peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB), injected into the lateral ventricle produce multiple effects such as hypothermia, anorexia and hormone release. In this study, the pharmacological characteristics of BN receptors mediating hypothermia in the central nervous system (CNS) were investigated using free-moving male Wistar rats. Intracerebroventricular injections of BN, GRP and NMB produced hypothermia in a dose-dependent manner. The BN (0.3 microg)-induced effect showed a short latency and a 4-h duration with a potency increased by more than 100 times compared to the NMB-induced effect. Pretreatment with [D-Tyr(6)]BN(6-13)methylester, a GRP receptor antagonist, inhibited the BN (0.3 microg)- and NMB (7 microg)-induced hypothermia. On the other hand, BIM23127, an NMB receptor antagonist, did not influence the hypothermia. Of the protein kinase C (PKC) inhibitors, chelerythrine, Go6983, staurosporine and GF109203X, the first two partially blocked the BN-induced hypothermia. A PKC activator, phorbol-12,13-dibutyrate, decreased the rectal temperature. Genistein (a tyrosine kinase inhibitor), Y-27632 (a Rho kinase inhibitor) and PD98059 (a MAPK inhibitor) tended to suppress the BN-induced hypothermia, however, these were not significant. The inhibitory effect of a mixture of the three inhibitors, chelerythrine, genistein and Y-27632, on the BN-induced hypothermia was of a similar degree to that of chelerythrine alone. The BN receptor mediating the hypothermia seem to be the GRP subtype, and the effect involves activation of PKC.

Identification and functional characterization of hemorphins VV-H-7 and LVV-H-7 as low-affinity agonists for the orphan bombesin receptor subtype 3

1. The human orphan G-protein coupled receptor bombesin receptor subtype 3 (hBRS-3) was screened for peptide ligands by a Ca(2+)mobilization assay resulting in the purification and identification of two specific ligands, the naturally occurring VV-hemorphin-7 (VV-H-7) and LVV-hemorphin-7 (LVV-H-7), from human placental tissue. These peptides were functionally characterized as full agonists with unique specificity albeit low affinity for hBRS-3 compared to other bombesin receptors. 2. VV-H-7 and LVV-H-7 induced a dose-dependent response in hBRS-3 overexpressing CHO cells, as well as in NCI-N417 cells expressing the hBRS-3 endogenously. The affinity of VV-H-7 was higher in NCI-N417 cells compared to overexpressing CHO cells. In detail, the EC(50) values were 45+/-15 microM for VV-H-7 and 183+/-60 microM for LVV-H-7 in CHO cells, and 19+/-6 microM for VV-H-7 and 38+/-18 microM for LVV-H-7 in NCI-N417 cells. Other hemorphins had no effect. Gastrin-releasing peptide (GRP) and neuromedin B (NMB) showed similar EC(50) values of 13-20 microM (GRP) and of 1-2 microM (NMB) on both cell lines. 3. Structure-function analysis revealed that both the N-terminal valine and the C-terminal phenylalanine residues of VV-H-7 are critical for the ligand-receptor interaction. 4. Endogenous hBRS-3 in NCI-N417 activated by VV-H-7 couples to phospholipase C resulting in changes of intracellular calcium, which is initially released from an inositol trisphosphate (IP(3))-sensitive store followed by a capacitive calcium entry from extracellular space. 5. VV-H-7-induced hBRS-3 activation led to phosphorylation of p42/p44-MAP kinase in NCI-N417 cells, but did not stimulate cell proliferation. In contrast, phosphorylation of focal adhesion kinase (p125(FAK)) was not observed.

Lithocholylcholine, a bile acid/acetylcholine hybrid, is a muscarinic receptor antagonist

Previous work from our laboratory indicates that bile acids, specifically lithocholic acid conjugates, interact with muscarinic receptors on gastric chief cells. Structural similarities between acetylcholine and lithocholyltaurine suggest a potential molecular basis for their interaction with the same receptor. We synthesized a hybrid molecule consisting of the steroid nucleus of lithocholyltaurine and the choline moiety of acetylcholine. The new molecule, lithocholylcholine, is hydrolyzed by acetyl-cholinesterase. Lithocholylcholine inhibited binding of a cholinergic radioligand to Chinese hamster ovary cells expressing each of the five muscarinic receptor subtypes. The binding affinities (K(i); micromolar) of lithocholylcholine for these receptors were: M3 (1.0) > M1 (2.7) > M2 (4.1) = M4 (4.9) > M5 (6.2). Lithocholylcholine inhibited intracellular signaling pathways mediated by interaction with M1, M2, and M3 muscarinic receptors. Regarding M3 receptors, lithocholylcholine was 10-fold more potent than lithocholyltaurine in terms of binding affinity and inhibition of acetylcholine-induced increases in inositol phosphate formation and mitogen-activated protein kinase phosphorylation. In a functional assay, lithocholylcholine inhibited acetylcholine-induced relaxation of rat aortic rings. These observations indicate that lithocholylcholine is a muscarinic receptor antagonist and provide further evidence that bile acids may have gastrointestinal signaling functions that extend beyond their effects on sterol metabolism, lipid absorption, and cholesterol elimination. Hybrid molecules created from bile acids and acetylcholine may be used to develop selective muscarinic receptor ligands.

Systematic optimization of a lead-structure identities for a selective short peptide agonist for the human orphan receptor BRS-3

The orphan receptor, human bombesin receptor subtype 3 (BRS-3) was assigned to the G-protein coupled bombesin receptor family because of its high sequence homology with the neuromedin B receptor (NMB-R) and gastrin-releasing peptide receptor (GRP-R). Since its pharmacology is stiIl unknown, new highly potent and selective tool-substances are needed, that may be able to elucidate its possible role in obesity and cancer. We have performed structure activity relationship studies on the high affinity peptide agonists [D-Phe6,beta-Ala11,Phe13,Nle14]Bn(6-14) and [D-Phe6,Phe13]Bn(6-13)propylamide, using their ability to mobilize intracellular calcium in BRS-3 transfected CHOGa-16 cells combined with receptor binding studies. It was demonstrated that for [D-Phe,beta-Ala11,Phe13,Nle14]Bn(6-14) the side chains of the residues Trp8 and Phe13, and to a smaller extent beta-Ala11, are the important amino acid side chains for receptor activation and binding, however for [D-Phe6,Phe13]Bn(6-13) propylamide His12 seems to be more important than Phe13. C-and N-terminal deletions and amino acid substitutions allowed further understanding. It was demonstrated that substitution of His 12 by Tyr leads to a high selectivity towards GRP-R. Using the acquired information, a small tetrapeptide library was designed with compounds presenting Trp and Phe at varying stereochemistry and distances, which led to the discovery of the lead-structure H-D-Phe-Gln-D-Trp-Phe-NH2. Systematic SAR revealed the important structural features of this peptide, C-terminal optimization resulted in the highly active and selective BRS-3 agonist H-D-Phe-Gln-D-Trp-1-(2-phenylethyl)amide. In summary, the size of the peptide was reduced from 8 or 9 amino acids to a tripeptide for BRS-3.

Deoxycholic acid conjugates are muscarinic cholinergic receptor antagonists

In the course of examining the actions of major human bile acids on cholinergic receptors, we discovered that conjugates of lithocholic acid are partial muscarinic agonists. In the present communication, we report that conjugates of deoxycholic acid (DC) act as cholinergic muscarinic receptor antagonists. Chinese hamster ovary (CHO) cells expressing rat M3-muscarinic receptors were used to test bile acids for inhibition of radioligand [N- (3)H-methylscopolamine ((3)H-NMS)] binding; alteration of inositol phosphate (IP) formation; mitogen-activated protein (MAP) kinase phosphorylation and cell toxicity. We observed approximately 18.8, 30.3 and 37.1% inhibition of (3)H-NMS binding with DC and its glycine (DCG) and taurine (DCT) conjugates, respectively (all 100 micromol/l, p < 0.01). DCT and DCG inhibited acetylcholine-induced increases in IP formation and MAP kinase phosphorylation (p44 and p42 ERK). DCG and DCT did not alter trypan blue exclusion or lactate dehydrogenase release from CHO-M3 cells. We observed the following rank order of potency (IC(50) micromol/l) for inhibition of (3)H-NMS by muscarinic antagonists and bile acids: NMS (0.0004) > 4-DAMP (0.009) > atropine (0.012) > DCT (170) > DCG (250). None of the bile acids tested were hydrolyzed by recombinant cholinesterase. At concentrations achieved in human bile, DC derivatives are natural muscarinic antagonists.

Discovery of potent and selective peptide agonists at the GRP-preferring bombesin receptor (BB2)

Analogues of the nonselective bombesin receptor synthetic agonist H-D-Phe-Gln-Trp-Ala-Val-betaAla-His-Phe-Nle-NH2 were prepared and their biological activity assessed at the NMB-preferring/bombesin receptor (NMB-R: BB1), the GRP-preferring/bombesin receptor (GRP-R: BB2) and the orphan receptor bombesin receptor subtype-3 (BRS-3; BB3). Progressive N-terminal deletions identified the minimum C-terminal sequences required for maintaining a significant agonist effect, whilst an alanine scan, targeted changes in stereochemistry and other pertinent substitutions identified key side-chain and stereochemical requirements for activation. Key structural elements required for functional potency at BB1 BB2 and BB3, and for selectivity between these receptor subtypes were established. Synthetic peptides were discovered. which were highly potent agonists at BB2 and extremely selective over both BB1 and BB3.

Rational design of a peptide agonist that interacts selectively with the orphan receptor, bombesin receptor subtype 3

The orphan receptor, bombesin (Bn) receptor subtype 3 (BRS-3), shares high homology with bombesin receptors (neuromedin B receptor (NMB-R) and gastrin-releasing peptide receptor (GRP-R)). This receptor is widely distributed in the central nervous system and gastrointestinal tract; target disruption leads to obesity, diabetes, and hypertension, however, its role in physiological and pathological processes remain unknown due to lack of selective ligands or identification of its natural ligand. We have recently discovered (Mantey, S. A., Weber, H. C., Sainz, E., Akeson, M., Ryan, R. R. Pradhan, T. K., Searles, R. P., Spindel, E. R., Battey, J. F., Coy, D. H., and Jensen, R. T. (1997) J. Biol. Chem. 272, 26062-26071) that [d-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]Bn-(6-14) has high affinity for BRS-3 and using this ligand showed BRS-3 has a unique pharmacology with high affinity for no known natural Bn peptides. However, use of this ligand is limited because it has high affinity for all known Bn receptors. In the present study we have attempted to identify BRS-3 selective ligands using a strategy of rational peptide design with the substitution of conformationally restricted amino acids into the prototype ligand [d-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]Bn-(6-14) or its d-Phe(6) analogue. Each of the 22 peptides synthesized had binding affinities determined for hBRS-3, hGRPR, and hNMBR, and hBRS-3 selective ligands were tested for their ability to activate phospholipase C and increase inositol phosphates ([(3)H]inositol phosphate). Using this approach we have identified a number of BRS-3 selective ligands. These ligands functioned as receptor agonists and their binding affinities were reflected in their potencies for altering [(3)H]inositol phosphate. Two peptides with an (R)- or (S)-amino-3-phenylpropionic acid substitution for beta-Ala(11) in the prototype ligand had the highest selectivity for the hBRS-3 over the mammalian Bn receptors and did not interact with receptors for other gastrointestinal hormones/neurotransmitters. Molecular modeling demonstrated these two selective BRS-3 ligands had a unique conformation of the position 11 beta-amino acid. This selectivity was of sufficient magnitude that these should be useful in explaining the role of hBRS-3 activation in obesity, glucose homeostasis, hypertension, and other physiological or pathological processes.

A bombesin receptor subtype-3 peptide increases nuclear oncogene expression in a MEK-1 dependent manner in human lung cancer cells

A synthetic peptide, (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) was used to investigate the signal transduction mechanisms of bombesin receptor subtype-3. Using NCI-1299#5 human lung cancer cells stably transfected with bombesin receptor subtype-3, 100 nM (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) elevated the cytosolic Ca2+ from 150 to 250 nM within 10 s. Addition of (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) caused phosphorylation of mitogen activated protein kinase in a time- and concentration-dependent manner. The mitogen activated protein kinase phosphorylation caused by (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) was inhibited by 2'-amino-3'-methyoxyflavone (PD98059), a mitogen activated protein kinase kinase (MEK-1) inhibitor. Using a luciferase reporter gene construct, (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) caused Elk-1 activation after 10 min and the increase in Elk-1 activation caused by (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) was inhibited by PD98059 as well as a dominant-negative MEK-1. (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) caused increased c-fos as well as c-jun mRNAs 1 h after addition to NCI-H1299#5 cells. The 47-fold increase in c-fos mRNA caused by 100 nM (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) was inhibited by PD98059, a dominant-negative MEK-1 and a substance P antagonist but not (3-phenylpropanoyl-D-Ala(24), Pro(26), Psi(26,27), Phe(27))GRP-(20-27) (BW2258U89), a GRP receptor antagonist. These results indicate that (D-Phe(6), beta-Ala(11), Phe(13), Nle(14))bombesin-(6-14) caused increased nuclear oncogene expression and upstream events include mitogen activated protein kinase phosphorylation and Elk-1 activation.

Neuromedin B

Neuromedin B (NMB) is one of the bombesin (BN)-related peptides in mammals. It was originally purified from pig spinal cords, and it has been shown to be present in central nervous system as well as in gastrointestinal tract. BN and its related peptides have various physiological effects. These include regulation of exocrine and endocrine secretions, smooth muscle contraction, feeding, blood pressure, blood glucose, body temperature and cell growth. NMB exerts its effect by binding to the cell surface receptor. A high affinity receptor, NMB receptor (NMB-R) has been identified. This is a G-protein coupled receptor with seven membrane-spanning regions. Upon agonist binding, several intracellular signaling cascades including phospholipase activation, calcium mobilization and protein kinase C (PKC) activation lead to expression of several genes, DNA synthesis or cellular effects such as secretion. Existence of NMB-R has been demonstrated in several brain regions, notably in olfactory and thalamic regions, and in gastrointestinal tracts. Recent analysis using NMB-R-deficient mice, generated by gene-targeting technique, enables to distinguish functional properties of NMB-R from GRP-R. In this review, molecular characterization, anatomical distribution and pharmacological properties of NMB and NMB-R will be presented. Moreover, physiological roles of NMB and its receptor demonstrated by the analysis of NMB-R-deficient mice will be reported. Comparison with GRP/GRP-R system will provide important information about BN-like peptide systems in mammals.

The presence of receptors for bombesin/GRP and mRNA for three receptor subtypes in human ovarian epithelial cancers

Bombesin-like peptides can function as autocrine or paracrine growth factors and stimulate the growth of various cancers. The antagonists of bombesin/gastrin-releasing peptide (GRP) suppress the proliferation of diverse tumors including ovarian cancer by mechanisms likely mediated by bombesin receptors. In this study, we used the reverse transcription-polymerase chain reaction (RT-PCR) method to evaluate the mRNA expression of three bombesin receptor subtypes: gastrin-releasing peptide receptor (GRPR), neuromedin B receptor (NMBR), and bombesin receptor subtype 3 (BRS-3), in 22 specimens of human epithelial ovarian cancer and in two human ovarian cancer lines. Of the 22 ovarian cancer specimens analyzed, 17 tumors ( approximately 77%) expressed mRNA for GRPR, 19 ( approximately 86%) showed NMBR mRNA and six ( approximately 27%) revealed BRS-3 mRNA. Thus, 14 of 22 specimens ( approximately 64%) expressed mRNAs for both GRPR and NMBR, and five ( approximately 23%) expressed all three subtypes. The expression of GRPR appeared to be greater in poorly differentiated ovarian carcinomas. A higher incidence of BRS-3 expression was observed in samples with tumor Stage IV (4/4, 100%) compared with Stage III (1/17, approximately 6%). mRNA for both GRPR and NMBR was also detected in OV-1063 and UCI-107 human ovarian cancer xenografts, but BRS-3 was found only in OV-1063, which originated from a metastatic tumor. In addition, functional receptors for bombesin/GRP were found in eight of 11 ovarian cancer specimens investigated and in both ovarian cancer lines by receptor binding assay. Our study indicates that GRPR and NMBR are widely distributed in human ovarian carcinomas with BRS-3 being found in Stage IV tumors. Some approaches based on bombesin/GRP receptor antagonists or targeted bombesin analogs could be considered for treatment of ovarian cancers.

(Tyr(0),Bpa(4))bombesin is a GRP receptor agonist

(Tyr(0),Bpa(4))bombesin, (YB)BB was synthesized and its biologic activity evaluated using T47D breast cancer cells. ((125)I-Tyr(0), Bpa(4))BB bound with high affinity (K(d) = 5 nM) to T47D cells. Specific ((125)I-Tyr(0),Bpa(4))BB binding was inhibited with high affinity by BB, BW2258U89, GRP, GRP(14-27) and NMB (IC(50) values of 10, 2, 15, 20, and 150 nM)but not GRP(1-16) (IC(50) value of > 1000 nM). ((125)I-Tyr(0),Bpa(4))BB bound to the surface of T47D cells at 4 degrees C but was internalized at 37 degrees C. After binding at 4 degrees C followed by irradiation using ultraviolet light, ((125)I-Tyr(0),Bpa(4))BB labeled a 75 kDa protein using T47D cells. (Tyr(0),Bpa(4))BB, 10 nM, elevated cytosolic calcium using T47D cells within 10 s. Also (Tyr(0),Bpa(4))BB, 10 nM, elevated c-fos mRNA after 45 min. These results indicate that (Tyr(0),Bpa(4))BB is an agonist for GRP receptors.

The metabolism of BW2258U89, a GRP receptor antagonist

BW2258U89 is a gastrin releasing peptide (GRP) receptor antagonist which inhibits the proliferation of the neuroendocrine tumor small cell lung cancer (SCLC). Here the biological activity of BW2258U89 and its metabolite were investigated. Using mass spectroscopy (LC-ESI/MS) techniques, three major peaks for BW2258U89 were observed with mass/charge (m/z) ratios of 1081.6, 541.4 and 361.4. After metabolism by mouse plasma enzymes, the major product had a m/z ratio of 1082.5, 541.9 and 361.8 suggesting that BW2258U89 was deamidated. Deamidated (Da) BW2258U89 was synthesized and it inhibited ((125)I-Tyr(4)) BB binding to NCI-H345 SCLC cells with an IC(50)value of 450 nM; BW2258U89 had an IC(50)value of 17 nM. BW2258U89 (1 microM) antagonized the ability of 50 nM BB to elevate cytosolic Ca(2+)in NCI-H345 cells, whereas 1 microM (Da) BW2258U89 did not. One micromolar BW2258U89 antagonized the increase in NCI-H345 c-fos mRNA caused by 10 nM BB, whereas 1 microM (Da) BW2258U89 had little effect. One microM BW2258U89 inhibited NCI-H345 clonal growth significantly whereas 1 microM (Da) BW2258U89 did not. These data suggest that an amidated C-terminal is important for antagonism of SCLC GRP receptors by BW2258U89.