Chronic desensitization and down-regulation of the gastrin-releasing peptide receptor are mediated by a protein kinase C-dependent mechanism

Bombesin Receptor Family Activation and CNS/Neural Tumors: Review of Evidence Supporting Possible Role for Novel Targeted Therapy

G-protein-coupled receptors (GPCRs) are increasingly being considered as possible therapeutic targets in cancers. Activation of GPCR on tumors can have prominent growth effects, and GPCRs are frequently over-/ectopically expressed on tumors and thus can be used for targeted therapy. CNS/neural tumors are receiving increasing attention using this approach. Gliomas are the most frequent primary malignant brain/CNS tumor with glioblastoma having a 10-year survival <1%; neuroblastomas are the most common extracranial solid tumor in children with long-term survival<40%, and medulloblastomas are less common, but one subgroup has a 5-year survival <60%. Thus, there is an increased need for more effective treatments of these tumors. The Bombesin-receptor family (BnRs) is one of the GPCRs that are most frequently over/ectopically expressed by common tumors and is receiving particular attention as a possible therapeutic target in several tumors, particularly in prostate, breast, and lung cancer. We review in this paper evidence suggesting why a similar approach in some CNS/neural tumors (gliomas, neuroblastomas, medulloblastomas) should also be considered.

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.

Function of non-visual arrestins in signaling and endocytosis of the gastrin-releasing peptide receptor (GRP receptor)

Little is known about the role of arrestins in gastrointestinal hormone/neurotransmitter receptor endocytosis. With other G protein-coupled receptors, arrestins induce G protein-uncoupling and receptor endocytosis. In this study, we used arrestin wild-type and dominant-negative mutant constructs to analyze the arrestin dependence of endocytosis and desensitization of the gastrin-releasing peptide receptor (GRP-R). Co-expression of the GRP-R with wild-type arrestin2 and arrestin3 increased not only GRP-R endocytosis but also GRP-R desensitization in arrestin-overexpressing cells. Co-expression of the dominant-negative mutants V53D-arrestin2 or V54D-arrestin3 reduced GRP-R endocytosis. Notably, different trafficking routes for agonist-activated GRP-R-arrestin2 and GRP-R-arrestin3 complexes were found. Arrestin3 internalizes with GRP-R to intracellular vesicles, arrestin2 splits from the GRP-R and localizes to the cell membrane. Also, the recycling pathway of the GRP-R was different if co-expressed with arrestin2 or arrestin3. Using different GRP-R mutants, the C-terminus and the 2nd intracellular loop of the GRP-R were found to be important for the GRP-R-arrestin interaction and for the difference in GRP receptor trafficking with the two arrestin subtypes. Our results show that both non-visual arrestins play an important role in GRP-R internalization and desensitization.

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.

Bombesin marine toxin conjugates inhibit the growth of lung cancer cells

Hemiasterlin (Hem) and dolastatin (Dol) are marine natural products which are cytotoxic for cancer cells. Hem, a tripeptide, and Dol, a hexapeptide, were conjugated with linkers (L) to the universal BB agonist DPhe-Gln-Trp-Ala-Val-betaAla-His-Phe-Nle-NH2(BA1) and the effects of the Hem-BB and Dol-BB conjugates investigated on NCI-H1299 lung cancer cells. Hem-LA-BA1 and Hem-LB-BA1 inhibited specific (125I-Tyr4)BB binding to NCI-H1299 cells, which have BB2 receptors (R), with IC50 values of 15 and 25 nM, respectively. Addition of Hem-LA-BA1 and Hem-LB-BA1 to Fura-2 AM loaded cells containing BB2R, caused elevated cytosolic Ca2+. In a growth assay, Hem-LA-BA1 and Hem-LB-BA1 inhibited the proliferation of NCI-H1299 cells. Dol-succinamide (Dols)-LD-BA1 and Dols-LE-BA1 bound with high affinity to NCI-H1299 cells and elevated cytosolic Ca2+, but did not inhibit the proliferation of NCI-H1299 cells. Also, Hem-LA-BA1 inhibited 125I-DTyr-Gln-Trp-Ala-Val-betaAla-His-Phe-Nle-NH2 (BA2) binding to Balb/3T3 cells transfected with BB1R or BB2R as well as with BRS-3 with IC50 values of 130, 8, and 540 nM, respectively. These results show that Hem-BB conjugates are cytotoxic for cancer cells containing BB2R.

Importance of amino acids of the central portion of the second intracellular loop of the gastrin-releasing Peptide receptor for phospholipase C activation, internalization, and chronic down-regulation

Little is known about the function of the central portion of the second intracellular loop (i2 loop) of peptide receptors in activation of downstream pathways and receptor modulatory processes such as receptor internalization or chronic down-regulation (DR). Recent data suggest a role for i2 loop hydrophobic amino acids in these processes. We used site-directed mutagenesis to address these issues with the gastrin-releasing peptide receptor (GRP-R). Each i2 loop residue from 142 to 148 was mutated and the receptors were expressed in Balb 3T3 cells. Two mutants showed a minimal (<2-fold) decrease in affinity. Five mutants showed decreased efficacy for activating phospholipase C (PLC). Two double mutants (IM143.147AA and VM144.147AA) showed a minimal decrease in affinity but had a decreased ability to fully activate PLC. Only the IM double mutation had decreased maximal internalization, whereas the R145A single mutant showed an increase, suggesting a tonic inhibitory role for Arg-145 in internalization. Three single and both double mutants showed decreases in receptor DR. There was a weak correlation between the extent of GRP-R internalization and the maximal PLC activation, whereas changes in the maximal PLC activation were significantly (p = 0.008) coupled to receptor DR. This study shows that amino acids of the i2 loop of the GRP-R are important in activation of PLC, internalization and down-regulation, but not for affinity. Our results support the proposal that internalization and chronic down-regulation have differing dependence on PLC and are largely independent processes, because some mutants showed no changes in internalization, but significant alterations in down-regulation.

Agonist- and protein kinase C-induced phosphorylation have similar functional consequences for gastrin-releasing peptide receptor signaling via Gq

Acute desensitization of many guanine nucleotide-binding protein-coupled receptors (GPCRs) requires receptor phosphorylation and subsequent binding of an arrestin. GPCRs are substrates for phosphorylation by several classes of kinases. Gastrin-releasing peptide receptor (GRPr) is phosphorylated by a kinase other than protein kinase C (PKC) after exposure to agonist and is also a substrate for PKC-dependent phosphorylation after treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA). Using GRPr mutants, we examined receptor domains required for agonist- and TPA-induced phosphorylation of GRPr and consequences of these phosphorylation events on GRPr signaling via Gq. Agonist- and TPA-stimulated GRPr phosphorylation in cells require an intact carboxyl terminal domain (CTD). GRPr is phosphorylated in vitro by GPCR kinase 2 (GRK2) and multiple PKC isoforms. An intact DRY motif is required for agonist-stimulated phosphorylation in cells, and agonist-dependent GRK2 phosphorylation in vitro. Although GRPr CTD mutants do not show enhanced in vitro coupling to Gq relative to intact GRPr, CTD mutants have more potent Gq-dependent signaling in cells. Acute desensitization involves CTD-independent processes because desensitization can precede ligand binding in intact GRPr and CTD mutants. TPA-mediated impairment of GRPr-Gq signaling in cells also requires an intact CTD. Similar to GRK2 phosphorylation, PKC phosphorylation reduces GRPr-Gq coupling by approximately 80% in vitro. Arrestin translocation to plasma membrane requires agonist, an intact DRY motif, and GRPr phosphorylation. Therefore, agonist- and PKC-induced GRPr phosphorylation sites are in nearby regions of the receptor, and phosphorylation at both sites has similar functional consequences for Gq signaling.

Protein kinase C-mediated desensitization of the neurokinin 1 receptor

An understanding of the mechanisms that regulate signaling by the substance P (SP) or neurokinin 1 receptor (NK1-R) is of interest because of their role in inflammation and pain. By using activators and inhibitors of protein kinase C (PKC) and NK1-R mutations of potential PKC phosphorylation sites, we determined the role of PKC in desensitization of responses to SP. Activation of PKC abolished SP-induced Ca(2+) mobilization in cells that express wild-type NK1-R. This did not occur in cells expressing a COOH-terminally truncated NK1-R (NK1-Rdelta324), which may correspond to a naturally occurring variant, or a point mutant lacking eight potential PKC phosphorylation sites within the COOH tail (NK1-R Ser-338, Thr-339, Ser-352, Ser-387, Ser-388, Ser-390, Ser-392, Ser-394/Ala, NK1-RKC4). Compared with wild-type NK1-R, the t(1/2) of SP-induced Ca(2+) mobilization was seven- and twofold greater in cells expressing NK1-Rdelta324 and NK1-RKC4, respectively. In cells expressing wild-type NK1-R, inhibition of PKC caused a 35% increase in the t(1/2) of SP-induced Ca(2+) mobilization. Neither inhibition of PKC nor receptor mutation affected desensitization of Ca(2+) mobilization to repeated challenge with SP or SP-induced endocytosis of the NK1-R. Thus PKC regulates SP-induced Ca(2+) mobilization by full-length NK1-R and does not regulate a naturally occurring truncated variant. PKC does not mediate desensitization to repeated stimulation or endocytosis of the NK1-R.

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

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

Glycosylation of the gastrin-releasing peptide receptor and its effect on expression, G protein coupling, and receptor modulatory processes

Many gastrointestinal G protein-coupled receptors are glycosylated; however, which potential glycosylation sites are actually glycosylated and their role in receptor transduction or receptor modulation (internalization, down-regulation, desensitization) is largely unknown. We used site-directed mutagenesis to address these issues with the gastrin-releasing peptide receptor (GRP-R). Each of the four potential glycosylation sites was mutated by converting the Asn (N) to Gln (Q). Transient expression in CHOP cells demonstrated that changing Asn(24) or Asn(191) inhibited GRP-R cell surface expression, whereas elimination of Asn(5) and Asn(20) had no effect. Using ligand cross-linking studies in stable mutants expressed in Balb 3T3 cells, all four potential extracellular sites were glycosylated with carbohydrate residues of approximately 13 kDa on Asn(5), 10 kDa on Asn(20), 5 kDa on Asn(24), and 9 kDa on Asn(191). Removal of three glycosylation sites (N5,20,24,Q mutant) did not alter receptor affinity or G protein coupling; therefore, it could be speculated that deglycosylation at Asn(191) might be responsible for the altered G protein coupling seen with complete enzymatic deglycosylation of the native receptor previously reported. Removal of any single glycosylation site did not interfere with GRP-R induced chronic desensitization or down-regulation. However, elimination of all three NH(2)-terminal sites (N5,20,24) markedly attenuated both processes, with no effect on acute homologous desensitization and with only a minimal alteration of GRP-R internalization, supporting the findings of other studies that suggest that chronic desensitization and down-regulation are functionally coupled, distinct from acute desensitization and distinct from internalization. These data show that separate and specific glycosylation sites are important for GRP-R trafficking to the cell surface, ligand binding, G protein coupling, chronic desensitization, and down-regulation.

Role for C-tail residues in delta opioid receptor downregulation

The delta opioid receptor, a member of the G-protein-coupled receptor superfamily, was used as a model system to characterize opioid receptor downregulation. Metabolic labeling followed by immunoprecipitation resulted in the isolation of the epitope-tagged mouse delta opioid receptor as a approximately 60-kDa protein. Prolonged agonist treatment with 100 nM d-Ala2, d-Leu5-enkephalin (DADLE) caused significant (approximately 60%) reduction in the level of receptor. The delta opioid receptor contains a number of phosphorylatable residues in the C tail. Point mutations of the majority of Ser/Thr sequences did not affect the level of downregulation, whereas mutation of Thr353 to Ala did. In order to test if phosphorylation at this site is involved in receptor downregulation, we generated a Thr353Glu mutant that would mimic the phosphorylated Thr at this site. This mutant exhibited a significantly higher extent of downregulation than the Thr353Ala mutant. In order to critically evaluate the requirement of Thr353 in receptor downregulation, we examined the downregulation of wildtype rat delta receptor (which does not contain Ala353) and an Ala353Thr point-mutant rat delta receptor. The wild-type receptor exhibited poor agonist-mediated downregulation, whereas Ala353Thr mutant exhibited increased downregulation. These results and results from additional studies with rat/mouse chimeric receptors support a role for phosphorylation of sites within the C tail in efficient downregulation of delta opioid receptors.

GnRH signalling pathways and GnRH-induced homologous desensitization in a gonadotrope cell line (alphaT3-1)

Exposure of the gonadotrope cells to gonadotropin-releasing hormone (GnRH) reduces their responsiveness to a new GnRH stimulation (homologous desensitization). The time frame as well as the mechanisms underlying this phenomenon are yet unclear. We studied in a gonadotrope cell line (alphaT3-1) the effects of short as well as long term GnRH pretreatments on the GnRH-induced phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities, by measuring the production of IP3, total inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that although rapid desensitization of GnRH-induced IP3 formation did not occur in these cells, persistent stimulation of cells with GnRH or its analogue resulted in a time-dependent attenuation of GnRH-elicited IPs formation. GnRH-induced IPs desensitization was potentiated after direct activation of PKC by the phorbol ester TPA, suggesting the involvement of distinct mechanisms in the uncoupling exerted by either GnRH or TPA on GnRH-stimulated PI hydrolysis. The levels of individual phosphoinositides remained unchanged under any desensitization condition applied. Interestingly, while the GnRH-induced PLA2 activity was rapidly desensitized (2.5 min) after GnRH pretreatments, the neuropeptide-evoked PLD activation was affected at later times, indicating an important time-dependent contribution of these enzymatic activities in the sequential events underlying the GnRH-induced homologous desensitization processes in the gonadotropes. Under GnRH desensitization conditions, TPA was still able to induce PLD activation and to further potentiate the GnRH-evoked PLD activity. AlphaT3-1 cells possess several PKC isoforms which, except PKCzeta, were differentially down-regulated by TPA (PKCalpha, betaII, delta, epsilon, eta) or GnRH (PKCbetaII, delta, epsilon, eta). In spite of the presence of PKC inhibitors or down-regulation of PKC isoforms by TPA, the desensitizing effect of the neuropeptide on GnRH-induced IPs, AA and PEt formation remained unchanged. In conclusion, in alphaT3-1 cells the GnRH-induced homologous desensitization affects the GnRH coupling with PLC, PLA2 and PLD by mechanism(s) which do not implicate TPA-sensitive PKC isoforms, but likely reflect time-dependent modification(s) on the activation processes of the enzymes.

Identification of a domain in the carboxy terminus of CCK receptor that affects its intracellular trafficking

The carboxy-terminal region of many guanine nucleotide-binding protein (G protein)-coupled receptors contains important regulatory sequences such as an NP(x)2-3Y motif, a site of fatty acid acylation, and serine- and threonine-rich domains. The type A CCK receptor contains all of these, yet their significance has not been examined. We have, therefore, constructed a series of receptor site mutants and truncations that interfere with each of these motifs and expressed each in Chinese hamster ovary cells where they were studied for radioligand binding, cell signaling, receptor internalization, and intracellular trafficking. Each construct was synthesized and transported appropriately to the cell surface, where CCK bound with high affinity, elicited an inositol 1,4, 5-trisphosphate response, and resulted in internalization and normal trafficking. Thus modification or elimination of each of these established sequence motifs had no substantial effect on any of these parameters of receptor and cellular function. However, an additional construct that truncated the carboxy terminus, eliminating an additional 15-amino-acid segment devoid of any currently recognized sequence motifs, resulted in a marked change in receptor trafficking, with all other parameters of receptor function normal. This mutant receptor construct was delayed at the stage of early endosomes, delaying its progress to the lysosome-enriched perinuclear compartment from the rapid time course followed by wild-type receptor and all of the other constructs. It is proposed that this region of the CCK receptor tail contains a new motif important for intracellular receptor trafficking.

Sustained bombesin exposure results in receptor down-regulation and tolerance to the chronic but not acute effects of bombesin on ingestion

Acute intracerebroventricular (i.c.v.) administration of bombesin (BN) reduces meal intake in fasted rats. The overall objective of the present study was to determine the behavioral and other ingestive effects of prolonged central administration of BN. In the first experiment, we characterized the effects of 8-day sustained central administration of BN (0, 0.01, 0.05, 0.1 and 5 micrograms/0.5 microliter/h) or its antagonist (BIM-26226; 0, 0.005, 0.05, 0.5 and 5.0 micrograms/0.5 microliter/h), in free feeding rats. Each dose was delivered over 48 h, in an ascending sequence. At the higher doses, the BN-exposed rats consumed significantly less food whereas those exposed to the BN antagonist ingested significantly more food than the controls (saline exposed), during the dark phase. Due to the limited 48-h exposure to these higher doses, we further investigated the effects of more sustained BN exposure. Thus BN was infused over a 7-day period, at a rate of 0.25 microgram BN/0.5 microliter/h. In this latter study, we determined the effects of sustained BN exposure on a) daily spontaneous ingestive pattern, b) the rat's ingestive and other behavioral responses to a subsequent acute BN (i.c.v.) challenge and, c) BN receptor binding profile in various brain regions. Over the initial 2 days of chronic infusion, BN significantly suppressed spontaneous ingestion, and this effect dissipated by 72 h. Upon acute challenge with bolus injection of BN (0.25 microgram; i.c.v.), both chronically BN-treated and control rats responded by decreased feeding and enhanced grooming behaviors. In terms of effects at the receptor level, chronic BN exposure resulted in significant down-regulation (reduced BN/GRP receptor density) at the PVN and the hippocampal dentate gyrus. These findings represent the first demonstration of concomitant behavioral and receptor based changes consequent to sustained exposure to BN. These data suggest that tolerance to feeding suppressant effects of BN develops gradually, which in part may be mediated by down-regulation of BN/GRP receptors at specific brain loci. Our results also suggest that despite the development of tolerance to the chronic or sustained effects of BN exposure, animals still respond robustly to acute fluctuations in peptide levels.

Endocytosis and recycling of G protein-coupled receptors

Agonist stimulation of G protein-coupled receptors causes a dramatic reorganization of their intracellular distribution. Activation of receptors triggers receptor endocytosis and, since receptors recycle back to the surface continuously, a new steady state is reached where a significant proportion of receptors is located internally. Although this movement of receptors is remarkable, its role has been enigmatic. Recent developments have provided insight into the compartments through which the receptors move, the nature of the signals that trigger receptor translocation, and the significance of receptor cycling for cell function. In this article, Jennifer Koenig and Michael Edwardson review recent progress in this field and place receptor cycling into a mathematical framework that reveals the extent and rate of intracellular receptor movement.

Effect of gastrin-releasing peptide receptor number on receptor affinity, coupling, degradation, and modulation

The relationship between receptor number and agonist-induced intracellular responses has been well studied in receptors coupled to adenylate cyclase; however, for receptors coupled to phospholipase C (PLC), very little is known about the effect of receptor number on receptor-mediated processes. To explore this issue, we investigated the effect of the number of receptors for gastrin-releasing peptide (GRP) on ligand affinity and on the ability to activate intracellular messengers [PLC, tyrosine phosphorylation of p125 focal adhesion kinase (p125FAK)] and cause receptor modulation (internalization, desensitization, down-regulation) and ligand degradation. Three BALB 3T3 cell lines were made that stably expressed the gastrin-releasing peptide receptor (GRP-R) with receptor numbers varying by 280-fold (GRP-R-Low, GRP-R-Med, and GRP-R-Hi). Each cell line had the same affinity for agonist. The efficacy for bombesin to increase [3H]inositol phosphates but not tyrosine phosphorylation of p125FAK correlated well with receptor number. In contrast, the EC50 value for [3H]inositol phosphate generation for bombesin was the same in each cell line. Receptor number did not alter internalization. In the absence of protease inhibitors, there was an inverse correlation between receptor number and receptor down-regulation and desensitization. However, with protease inhibitors present, GRP-R-Med and GRP-R-Hi down-regulated significantly less than the GRP-R-Low. Similarly, GRP-R-Low desensitized significantly more than GRP-R-Med or GRP-R-Hi. GRP-R-Hi caused significantly greater ligand degradation than GRP-R-Low, and protease inhibitors completely inhibited degradation by GRP-R-Low and inhibited degradation by 70% for GRP-R-Hi. In conclusion, we show that for the PLC-coupled GRP-R, receptor number had little or no effect on binding affinity, potency for activating PLC, tyrosine phosphorylation of p125FAK, or extent of receptor internalization. In contrast, receptor number had an effect on ligand degradation, down-regulation, desensitization, and efficacy of PLC activation without altering the efficacy of tyrosine phosphorylation of p125FAK. These results demonstrate that the effect of receptor number differs for the different functions mediated by the GRP receptor and differs from that reported for adenylate cyclase-coupled receptors such as receptors mediating the action of adrenergic agents, secretin, and opioids.

Thr353, located within the COOH-terminal tail of the delta opiate receptor, is involved in receptor down-regulation

Prolonged exposure to abused drugs such as opiates causes decreased response to the drug; this reduced sensitivity is thought to be due to the loss of receptors, or down-regulation. The molecular mechanism of the opiate receptor down-regulation is not known. In order to address this, we generated a number of mutants of the delta opiate receptor COOH-terminal tail. When expressed in the Chinese hamster ovary cells, both the wild type and the receptor with a deletion of 37 COOH-terminal residues bind diprenorphine with comparable affinities and show similar decreases in cAMP levels in response to D-Ala2, D-Leu5, enkephalin (DADLE). However, the truncated receptor does not show down-regulation from the cell surface upon prolonged exposure (2-48 h) to DADLE. In contrast, both the wild type receptor and the receptor with the deletion of only 15 COOH-terminal residues show substantial down-regulation upon long term DADLE treatment. These results suggest that the region located between 15 and 37 residues from the COOH terminus is involved in the receptor down-regulation. In order to identify residues that play a key role in down-regulation, point mutations of residues within this region were examined for their ability to modulate receptor down-regulation. The receptor with a mutation of Thr353 to Ala does not down-regulate, whereas the receptor with a mutation of Ser344 to Gly down-regulates with a time course similar to that of the wild type receptor. Taken together, these results suggest that the COOH-terminal tail is not essential for functional coupling but is necessary for down-regulation and that Thr353 is critical for the agonist-mediated down-regulation of the delta opiate receptor.

Comparison of the peptide structural requirements for high affinity interaction with bombesin receptors

Recently it has been established that both a gastrin-releasing peptide (GRP)-preferring bombesin receptor and a neuromedin B-preferring bombesin receptor mediate the mammalian actions of bombesin-related peptides. Because many tissues used for studies of the structure-activity relationship of these peptides possess both receptor subtypes and none possess only the neuromedin B-preferring subtype, there is minimal information on the peptide structural features determining receptor selectivity and it is unknown whether the determinants of agonism at both bombesin receptor subtypes are similar. In the present study we have used native cells either possessing only one bombesin receptor subtype or stably transfected with one subtype to study in detail the peptide structural requirements for interacting and activating each receptor subtype. For the naturally occurring agonists, at the GRP-preferring bombesin receptor the relative affinities were litorin = ranatensin = bombesin > GRP >> neuromedin B, phyllolitorin and at the neuromedin B-preferring bombesin receptor were litorin = neuromedin B = ranatensin > bombesin, phyllolitorin >> GRP. For the GRP-preferring bombesin receptor the heptapeptide and for the neuromedin B-preferring bombesin receptor the octapeptide was the minimal carboxyl fragment interacting with the receptor/or causing biologic activity, and the nonapeptide and full decapeptide, respectively, were the minimal required for full affinity. Making neuromedin B more bombesin- or GRP-like by replacing amino acids in position 3, 6, and 9 demonstrated that position 3 was the most important, followed by position 9 for receptor subtype selectivity. A conformationally restricted GRP analogue, [D-Cys6,D-Ala11,Cys14]bombesin-(6-14) had a significantly higher affinity for GRP-preferring bombesin receptor than NMB receptor. These results demonstrate that: (1) the structure-function relations for the two mammalian bombesin receptors have important differences; (2) suggest that the active conformation of neuromedin B must differ markedly from the beta-sheet model proposed for GRP; and (3) suggest that one important function of the NH2 terminus of GRP and neuromedin B is determining receptor subtype selectivity.