Vasoactive intestinal peptide (VIP)1 receptor. Three nonadjacent amino acids are responsible for species selectivity with respect to recognition of peptide histidine isoleucineamide

Functional Expression of IP, 5-HT4, D1, A2A, and VIP Receptors in Human Odontoblast Cell Line

Odontoblasts are involved in sensory generation as sensory receptor cells and in dentin formation. We previously reported that an increase in intracellular cAMP levels by cannabinoid 1 receptor activation induces Ca²⁺ influx via transient receptor potential vanilloid subfamily member 1 channels in odontoblasts, indicating that intracellular cAMP/Ca²⁺ signal coupling is involved in dentinal pain generation and reactionary dentin formation. Here, intracellular cAMP dynamics in cultured human odontoblasts were investigated to understand the detailed expression patterns of the intracellular cAMP signaling pathway activated by the G_s protein-coupled receptor and to clarify its role in cellular functions. The presence of plasma membrane Gα_s as well as prostaglandin I₂ (IP), 5-hydroxytryptamine 5-HT₄ (5-HT₄), dopamine D₁ (D₁), adenosine A_2A (A_2A), and vasoactive intestinal polypeptide (VIP) receptor immunoreactivity was observed in human odontoblasts. In the presence of extracellular Ca²⁺, the application of agonists for the IP (beraprost), 5-HT₄ (BIMU8), D₁ (SKF83959), A_2A (PSB0777), and VIP (VIP) receptors increased intracellular cAMP levels. This increase in cAMP levels was inhibited by the application of the adenylyl cyclase (AC) inhibitor SQ22536 and each receptor antagonist, dose-dependently. These results suggested that odontoblasts express G_s protein-coupled IP, 5-HT₄, D₁, A_2A, and VIP receptors. In addition, activation of these receptors increased intracellular cAMP levels by activating AC in odontoblasts.

Targeting VIP and PACAP Receptor Signaling: New Insights into Designing Drugs for the PACAP Subfamily of Receptors

Pituitary Adenylate Cyclase-Activating Peptide (PACAP) and Vasoactive Intestinal Peptide (VIP) are neuropeptides involved in a diverse array of physiological and pathological processes through activating the PACAP subfamily of class B1 G protein-coupled receptors (GPCRs): VIP receptor 1 (VPAC1R), VIP receptor 2 (VPAC2R), and PACAP type I receptor (PAC1R). VIP and PACAP share nearly 70% amino acid sequence identity, while their receptors PAC1R, VPAC1R, and VPAC2R share 60% homology in the transmembrane regions of the receptor. PACAP binds with high affinity to all three receptors, while VIP binds with high affinity to VPAC1R and VPAC2R, and has a thousand-fold lower affinity for PAC1R compared to PACAP. Due to the wide distribution of VIP and PACAP receptors in the body, potential therapeutic applications of drugs targeting these receptors, as well as expected undesired side effects, are numerous. Designing selective therapeutics targeting these receptors remains challenging due to their structural similarities. This review discusses recent discoveries on the molecular mechanisms involved in the selectivity and signaling of the PACAP subfamily of receptors, and future considerations for therapeutic targeting.

Chemotactic activities of vasoactive intestinal peptide, neuropeptide Y and substance P in Leishmania braziliensis

Cell-cell interaction and active migration (and invasion) of parasites into skin host-cell(s) are key steps for successful infection by Leishmania. Chemotaxis constitutes a primordial chapter of Leishmania-host cell interaction, potentially modulated by neuropeptides released into the skin due, for example, to the noxious stimuli represented by the insect bite. Herein we have evaluated in vitro the effect of sensory (Substance P, SP) and autonomic (Vasoactive Intestinal Peptide, VIP, and Neuropeptide Y, NPY) neuropeptides on parasite taxis, and investigated the potential modulatory effect of SP on Leishmania (Viannia) braziliensis-macrophage interaction. We demonstrated that VIP (10^-10 M) and NPY (10^-9 M) are chemorepellent to the parasites, while SP (10^-8 M) produces a chemoattractant response. SP did not affect macrophage viability but seems to impair parasite-macrophage interaction as it decreased promastigote adherence to macrophages. As this effect is blocked by ([D-Pro 2, D-Trp7,9]-Substance P (10^-6 M), the observed action may be mediated by neurokinin-1 (NK₁) transmembrane receptors. VIP and NPY repellent chemotactic effect is impaired by their corresponding receptor antagonists. Additionally, they suggest that SP may be a key molecule to guide promastigote migration towards, and interaction, with dendritic cells and macrophage host cells.

Structural and functional insights into the juxtamembranous amino-terminal tail and extracellular loop regions of class B GPCRs

Class B guanine nucleotide-binding protein GPCRs share heptahelical topology and signalling via coupling with heterotrimeric G proteins typical of the entire superfamily of GPCRs. However, they also exhibit substantial structural differences from the more extensively studied class A GPCRs. Even their helical bundle region, most conserved across the superfamily, is predicted to differ from that of class A GPCRs. Much is now known about the conserved structure of the amino-terminal domain of class B GPCRs, coming from isolated NMR and crystal structures, but the orientation of that domain relative to the helical bundle is unknown, and even less is understood about the conformations of the juxtamembranous amino-terminal tail or of the extracellular loops linking the transmembrane segments. We now review what is known about the structure and function of these regions of class B GPCRs. This comes from indirect analysis of structure-function relationships elucidated by mutagenesis and/or ligand modification and from the more direct analysis of spatial approximation coming from photoaffinity labelling and cysteine trapping studies. Also reviewed are the limited studies of structure of some of these regions. No dominant theme was recognized for the structures or functional roles of distinct regions of these juxtamembranous portions of the class B GPCRs. Therefore, it is likely that a variety of molecular strategies can be engaged for docking of agonist ligands and for initiation of conformational changes in these receptors that would be expected to converge to a common molecular mechanism for activation of intracellular signalling cascades.

Strategies for studying the ligand binding site of GPCRs: photoaffinity labeling of the VPAC1 receptor, a prototype of class B GPCRs

G protein-coupled receptors (GPCRs) are crucial receptors acting as molecular sensors for many physiological and pathological processes. Class B GPCRs represent a small GPCR subfamily encompassing 15 members, and are very promising targets for the development of drugs to improve many diseases such as chronic inflammation, neurodegeneration, diabetes, stress, and osteoporosis. Over the past decade, structure-function relationship studies have demonstrated that the N-terminal ectodomain (N-ted) of class B GPCRs plays a pivotal role in natural ligand recognition. The N-ted structure of some class B GPCRs folds into a Sushi domain consisting of two antiparallel β sheets stabilized by three disulfide bonds and a salt bridge. The VPAC1 receptor is an archetype of class B GPCRs that binds vasoactive intestinal peptide (VIP), a neuropeptide modulating many physiological processes. The structure-function relationship of VPAC1 has been extensively studied. The use of a photoaffinity labeling strategy has been a powerful approach to determine the physical contacts between the functional receptor and its ligand. Those studies, coupled with 3D molecular modeling techniques, have clearly demonstrated the crucial role of the VPAC1 receptor N-ted in VIP recognition.

VPAC receptors: structure, molecular pharmacology and interaction with accessory proteins

The vasoactive intestinal peptide (VIP) is a neuropeptide with wide distribution in both central and peripheral nervous systems, where it plays important regulatory role in many physiological processes. VIP displays a large biological functions including regulation of exocrine secretions, hormone release, fetal development, immune responses, etc. VIP appears to exert beneficial effect in neuro-degenerative and inflammatory diseases. The mechanism of action of VIP implicates two subtypes of receptors (VPAC1 and VPAC2), which are members of class B receptors belonging to the super-family of GPCR. This article reviews the current knowledge regarding the structure and molecular pharmacology of VPAC receptors. The structure-function relationship of VPAC1 receptor has been extensively studied, allowing to understand the molecular basis for receptor affinity, specificity, desensitization and coupling to adenylyl cyclase. Those studies have clearly demonstrated the crucial role of the N-terminal ectodomain (N-ted) of VPAC1 receptor in VIP recognition. By using different approaches including directed mutagenesis, photoaffinity labelling, NMR, molecular modelling and molecular dynamic simulation, it has been shown that the VIP molecule interacts with the N-ted of VPAC1 receptor, which is itself structured as a 'Sushi' domain. VPAC1 receptor also interacts with a few accessory proteins that play a role in cell signalling of receptors. Recent advances in the structural characterization of VPAC receptor and more generally of class B GPCRs will lead to the design of new molecules, which could have considerable interest for the treatment of inflammatory and neuro-degenerative diseases.

The VPAC1 receptor: structure and function of a class B GPCR prototype

The class B G protein-coupled receptors (GPCRs) represents a small sub-family encompassing 15 members, and are very promising targets for the development of drugs to treat many diseases such as chronic inflammation, neurodegeneration, diabetes, stress, and osteoporosis. The VPAC1 receptor which is an archetype of the class B GPCRs binds Vasoactive Intestinal Peptide (VIP), a neuropeptide widely distributed in central and peripheral nervous system modulating many physiological processes including regulation of exocrine secretions, hormone release, foetal development, immune response … VIP appears to exert beneficial effect in neurodegenerative and inflammatory diseases. This article reviews the current knowledge regarding the structure and molecular pharmacology of VPAC1 receptors. Over the past decade, structure-function relationship studies have demonstrated that the N-terminal ectodomain (N-ted) of VPAC1 plays a pivotal role in VIP recognition. The use of different approaches such as directed mutagenesis, photoaffinity labeling, Nuclear Magnetic Resonance (NMR), molecular modeling, and molecular dynamic simulation has led to demonstrate that: (1) the central and C-terminal part of the VIP molecule interacts with the N-ted of VPAC1 receptor which is itself structured as a « Sushi » domain; (2) the N-terminal end of the VIP molecule interacts with the first transmembrane domain of the receptor where three residues (K(143), T(144), and T(147)) play an important role in VPAC1 interaction with the first histidine residue of VIP.

Molecular basis for the selectivity of the mammalian bombesin peptide, neuromedin B, for its receptor

The mammalian bombesin (Bn) peptides, neuromedin B (NMB) and gastrin-releasing peptide (GRP), have widespread actions in many tissues, and their effects are mediated by two closely related G-protein-coupled receptors, the NMBR and GRPR. Little is known about the structural determinants of NMBR selectivity for NMB, in contrast to GRP selectivity for the GRPR, which has been extensively studied. To provide insight, chimeric NMBR-GRPR loss-of-affinity and gain-of-affinity mutants were made, as well as NH(2)-terminally truncated NMBR and point mutants using site-directed mutagenesis. Receptors were expressed in Balb-3T3-cells or CHOP cells, and affinities were determined. NMB had 115-fold greater affinity for NMBR than GRPR. Receptor-chimeric studies showed that NMBR selectivity for NMB was primarily determined by differences in the third extracellular (EC3) regions of GRPR-NMBR and adjacent upper-transmembrane-5 (TM5) region. In this region, 24 NMB gain-of-affinity GRPR mutants or NMBR loss-of-affinity point/combination mutants were made. Three gain-of-affinity mutant GRPRs [[A198I] (EC3), [H202Q] (EC3), [S215I] (upper TM5)] had increased NMB affinity (2.4-21-fold), and these results were confirmed with NMBR loss-of-affinity mutants [I199A,Q203H,I215S-NMBR]. The combination mutant [A198I,S215]GRPR had the greatest effect causing a complete NMB gain-of-affinity. The importance of differences at position 199NMBR or 203NMBR was studied by substituting amino acids with various properties. Our results show that NMBR selectivity for NMB is due to differences in the EC3 of NMBR-GRPR and the adjacent upper-TM5 region. Within these regions, isoleucines in NMBR [position 199 (EC3)] (instead of A198GRPR) and in 215NMBR (TM5) (instead of S214GRPR), as well as Q203NMBR (instead of H202GRPR) are responsible for high NMB-affinity/selectivity of NMBR. The effect at position 199 is primarily due to differences in hydrophobicity of the substitution, whereas steric factors and charge of the substitution at position 203 were important determinants of NMB selectivity.

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.

Unconventional binding sites and receptors for VIP and related peptides PACAP and PHI/PHM: an update

The 28-amino-acid neuropeptide VIP and related peptides PACAP and PHI/PHM modulate virtually all of the vital functions in the body. These peptides are also commonly recognized as major regulators of cell growth and differentiation. Through their trophic and cytoprotective functions, they appear to play major roles in embryonic development, neurogenesis and the progression of a number of cancer types. These peptides bind to three well-characterized subtypes of G-protein coupled receptors: VPAC1 and VPAC2 share a common high affinity in the nanomolar range for VIP and PACAP; a third receptor type, PAC1, has been characterized for its high affinity for PACAP but its low affinity for VIP. Complex effects and pharmacological behaviors of these peptides suggest that multiple subtypes of binding sites may cooperate to mediate their function in target cells and tissues. In this complex response, some of these binding sites correspond to the definition of the conventional receptors cited above, while others display unexpected pharmacological and functional properties. Here we present potential clues that may lead investigators to further characterize the molecular nature and functions of these atypical binding species.

Peptide agonist docking in the N-terminal ectodomain of a class II G protein-coupled receptor, the VPAC1 receptor. Photoaffinity, NMR, and molecular modeling

The neuropeptide vasoactive intestinal peptide (VIP) strongly impacts on human pathophysiology and does so through interaction with class II G protein-coupled receptors named VIP pituitary adenylate cyclase-activating peptide (PACAP) receptors (VPACs). The molecular nature of VIP binding to receptors remains elusive. In this work, we have docked VIP in the human VPAC1 receptor by the following approach. (i) VIP probes containing photolabile residues in positions 6, 22, and 24 of VIP were used to photolabel the receptor. After receptor cleavage and Edman sequencing of labeled receptor fragments, it was shown that Phe6, Tyr22, and Asn24 of VIP are in contact with Asp107, Gly116, and Cys122 in the N-terminal ectodomain (N-ted) of the receptor, respectively. (ii) The structure of VIP was determined by NMR showing a central alpha helix, a disordered N-terminal His1-Phe6 segment and a 3(10) Ser25-Asn28 helix termination. (iii) A three-dimensional model of the N-ted of hVPAC1 was constructed by using the NMR structure of the N-ted of corticotropin-releasing factor receptor 2beta as a template. As expected, the fold is identified as a short consensus repeat with two antiparallel beta sheets and is stabilized by three disulfide bonds. (iv) Taking into account the constraints provided by photoaffinity, VIP was docked into the hVPAC1 receptor N-ted. The 6-28 fragment of VIP nicely lies in the N-ted C-terminal part, but the N terminus region of VIP is free for interacting with the receptor transmembrane region. The data provide a structural rationale to the proposed two-step activation mechanism of VPAC receptor and more generally of class II G protein-coupled receptors.

The significance of vasoactive intestinal peptide in immunomodulation

First identified by Said and Mutt some 30 years ago, the vasoactive intestinal peptide (VIP) was originally isolated as a vasodilator peptide. Subsequently, its biochemistry was elucidated, and within the 1st decade, their signature features as a neuropeptide became consolidated. It did not take long for these insights to permeate the field of immunology, out of which surprising new attributes for VIP were found in the last years. VIP is rapidly transforming into something more than a mere hormone. In evolving scientifically from a hormone to a novel agent for modifying immune function and possibly a cytokine-like molecule, VIP research has engaged many physiologists, molecular biologists, biochemists, endocrinologists, and pharmacologists and it is a paradigm to explore mutual interactions between neural and neuroendocrine links in health and disease. The aim of this review is firstly to update our knowledge of the cellular and molecular events relevant to VIP function on the immune system and secondly to gather together recent data that support its role as a type 2 cytokine. Recognition of the central functions VIP plays in cellular processes is focusing our attention on this "very important peptide" as exciting new candidates for therapeutic intervention and drug development.

VIP enhances both pre- and postsynaptic GABAergic transmission to hippocampal interneurones leading to increased excitatory synaptic transmission to CA1 pyramidal cells

Vasoactive intestinal peptide (VIP) is present in the hippocampus in three subtypes of GABAergic interneurones, two of which innervate preferentially other interneurones, responsible for pyramidal cell inhibition. We investigated how pre- and postsynaptic modulation of GABAergic transmission (to both pyramidal cells and interneurones) by VIP could influence excitatory synaptic transmission in the CA1 area of the hippocampus. VIP (0.1-100 nM) increased [(3)H]GABA release from hippocampal synaptosomes (maximum effect at 1 nM VIP; 63.8 +/- 4.0%) but did not change [(3)H]glutamate release. VIP (0.3-30 nM) enhanced synaptic transmission in hippocampal slices (maximum effect at 1 nM VIP; field excitatory postsynaptic potentials (epsp) slope: 23.7 +/- 1.1%; population spike amplitude: 20.3 +/- 1.7%). The action on field epsp slope was fully dependent on GABAergic transmission since it was absent in the presence of picrotoxin (50 microM) plus CGP55845 (1 microM). VIP (1 nM) did not change paired-pulse facilitation but increased paired-pulse inhibition in CA1 pyramidal cells (16.0 +/- 0.9%), reinforcing the involvement of GABAergic transmission in the action of VIP. VIP (1 nM) increased muscimol-evoked inhibitory currents by 36.4 +/- 8.7% in eight out of ten CA1 interneurones in the stratum radiatum. This suggests that VIP promotes increased inhibition of interneurones that control pyramidal cells, leading to disinhibition of synaptic transmission to pyramidal cell dendrites. In conclusion, concerted pre- and postsynaptic actions of VIP lead to disinhibition of pyramidal cell dendrites causing an enhancement of synaptic transmission.

Diffuse pharmacophoric domains of vasoactive intestinal peptide (VIP) and further insights into the interaction of VIP with the N-terminal ectodomain of human VPAC1 receptor by photoaffinity labeling with [Bpa6]-VIP

The widespread 28-amino acid neuropeptide vasoactive intestinal peptide (VIP) exerts its many biological effects through interaction with serpentine class II G protein-coupled receptors named VPAC receptors. We previously provided evidence for a physical contact between the side chain at position 22 of VIP and the N-terminal ectodomain of the hVPAC1 receptor (Tan, Y. V., Couvineau, A., Van Rampelbergh, J., and Laburthe, M. (2003) J. Biol. Chem. 278, 36531-36536). We explored here the contact site between hVPAC1 receptor and the side chain at position 6 of VIP by photoaffinity labeling. The photoreactive para-benzoyl-l-Phe (Bpa) was substituted for Phe(6) in VIP resulting in [Bpa(6)]-VIP, which was shown to be a hVPAC1 receptor agonist in Chinese hamster ovary cells stably expressing the recombinant receptor. After obtaining the covalent (125)I-[Bpa(6)-VIP].hVPAC1 receptor complex, it was sequentially cleaved by cyanogen bromide, peptide N-glycosidase F, endopeptidase Glu-C, and trypsin, and the cleavage products were analyzed by electrophoresis. The data demonstrated that (125)I-[Bpa(6)-VIP] were covalently attached to the short 104-108 fragment within the N-terminal ectodomain of the receptor. The data were confirmed by creation of a receptor mutant with new CNBr cleavage site. In a three-dimensional model of the receptor N-terminal ectodomain, this fragment was located on one edge of the putative VIP-binding groove and was adjacent to the fragment covalently attached to the side chain at position 22 of VIP. Altogether these data showed that the central part of VIP, at least between Phe(6) and Tyr(22), interacts with the N-terminal ectodomain of the hVPAC1 receptor.

Photoaffinity labeling demonstrates physical contact between vasoactive intestinal peptide and the N-terminal ectodomain of the human VPAC1 receptor

Vasoactive intestinal peptide (VIP) is a prominent neuropeptide whose actions are mediated by VPAC receptors belonging to class II G protein-coupled receptors. To identify contact sites between VIP and its VPAC1 receptor, an analog of VIP substituted with a photoreactive para-benzoyl-l-Phe (Bpa) at position 22 has been synthesized and evaluated in Chinese hamster ovary cells stably expressing the recombinant human receptor. Bpa22-VIP and native VIP are equipotent in stimulating adenylyl cyclase activity in cell membranes. Cyanogen bromide cleavage of the covalent 125I-[Bpa22-VIP]-hVPAC1R complex yielded a single labeled fragment of 30 kDa that shifted to 11 after deglycosylation, most consistent with the 67-137 fragment of the receptor N-terminal ectodomain. Further cleavage of this fragment with V8 endoproteinase and creation of receptor mutants with new CNBr cleavage sites (XàMet), demonstrated that 125I-[Bpa22-VIP] was covalently attached to the short receptor 109-120 fragment (GWTHLEPGPYPI). In a three-dimensional model of the receptor N-terminal ectodomain, this fragment is located on one edge of the putative VIP binding groove and encompasses several amino acids previously shown to be crucial for VIP binding (reviewed in Laburthe, M., Couvineau, A., and Marie, J. C. (2002) Receptors Channels 8, 137-153). Our data provide the first direct evidence for a physical contact between VIP and the N-terminal ectodomain of the hVPAC1 receptor.

Identification of cytoplasmic domains of hVPAC1 receptor required for activation of adenylyl cyclase. Crucial role of two charged amino acids strictly conserved in class II G protein-coupled receptors

The VPAC1 receptor mediates the action of two neuropeptides, vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide. It is a class II G protein-coupled receptor-activating adenylyl cyclase (AC). The role of the N-terminal extracellular domain of hVPAC1 receptor for VIP binding is now established (Laburthe, M., Couvineau, A. and Marie, J. C. (2002) Recept. Channels 8, 137-153), but nothing is known regarding the cytoplasmic domains responsible for AC activation. Here, we constructed a large series of mutants by substituting amino acids with alanine in the intracellular loops (IL) 1, 2, and 3 and proximal C-terminal tail of the receptor. The mutation of 40 amino acids followed by expression of mutants in chinese hamster ovary cells showed the following. (i) Mutations IL1 result in the absence of expression of mutants, suggesting a role of this loop in receptor folding. (ii) All residues of IL2 can be mutated without alteration of receptor expression and AC response to VIP. (iii) Mutation of residues IL3 points to the specific role of lysine 322 in the efficacy of the stimulation of AC activity by VIP. This efficacy is reduced by 50% in the K322A mutant. (iv) The proximal C-terminal tail is equipped with another important amino acid since mutation of glutamic acid 394 reduces AC response by 50%. The double mutant K322A/E394A exhibits a drastic reduction of >85% in the efficacy of VIP in stimulating AC activity in membranes and cAMP response in intact cells without alteration of receptor expression or affinity for VIP. These data highlight the role of charged residues in IL3 and the proximal C-terminal tail of hVPAC1 receptor for agonist-induced AC activation. Because these charged residues are absolutely conserved in class II receptors for peptides, which are all mediating AC activation, they may play a general role in coupling of class II receptors with the Gs protein.

Evidence for a direct interaction between the Thr11 residue of vasoactive intestinal polypeptide and Tyr184 located in the first extracellular loop of the VPAC2 receptor

We developed previously VPAC(1) [vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP) receptor]>VPAC(2) receptor selective ligands. Replacement of the VIP-Thr(11) by an Arg(11) in these ligands contributed to their selectivity: Arg(11)-VIP had a 200-fold lower affinity when compared with VIP at VPAC(2) receptors as opposed to 3- to 5-fold higher affinity at VPAC(1) receptors. Comparison of the binding and functional properties of related VIP analogues suggested that the VPAC(1) selectivity of Arg(11)-VIP was due to the loss of a hydrogen bond between the hydroxy group of Thr residue and the VPAC(2) receptor, steric hindrance between the Arg side chain and the VPAC(2) receptor and charge attraction by the VPAC(1) receptor. Comparison of the ability of VIP analogues to activate adenylate cyclase through chimaeric VPAC(1)/VPAC(2) and VPAC(2)/VPAC(1) receptors indicated that the first extracellular receptor loop carried most of the VPAC(2) receptors' ability to discriminate VIP from Arg(11)-VIP. Based on results obtained for a truncated VPAC(2) receptor and the closely related PACAP-preferring receptor (PAC(1)) and secretin receptors, we hypothesized that Thr(11) interacted with the VPAC(2) receptor Tyr(184) (similar to the VPAC(1) receptor Phe(200) residue). The Y184F (Tyr(184)-->Phe) VPAC(2) mutant lost the ability to discriminate VIP from Val(11)-VIP, and the F200Y VPAC(1) mutant acquired the ability to discriminate the natural peptide from Val(11)-VIP. These results support the hypothesis that the hydroxy group of the native VIP-Thr(11) side chain can indeed form a hydrogen bond with the Tyr side chain in the VPAC(2) receptor.

Different domains of the glucagon and glucagon-like peptide-1 receptors provide the critical determinants of ligand selectivity

(1) Glucagon and glucagon-like peptide-1 (GLP-1) are homologous peptide hormones with important functions in glucose metabolism. The receptors for glucagon and GLP-1 are homologous family B G-protein coupled receptors. The GLP-1 receptor amino-terminal extracellular domain is a major determinant of glucagon/GLP-1 selectivity of the GLP-1 receptor. However, the divergent residues in glucagon and GLP-1 that determine specificity for the GLP-1 receptor amino-terminal extracellular domain are not known. Less is known about how the glucagon receptor distinguishes between glucagon and GLP-1. (2) We analysed chimeric glucagon/GLP-1 peptides for their ability to bind and activate the glucagon receptor, the GLP-1 receptor and chimeric glucagon/GLP-1 receptors. The chimeric peptide GLP-1(7-20)/glucagon(15-29) was unable to bind and activate the glucagon receptor. Substituting the glucagon receptor core domain with the GLP-1 receptor core domain (chimera A) completely rescued the affinity and potency of GLP-1(7-20)/glucagon(15-29) without compromising the affinity and potency of glucagon. Substituting transmembrane segment 1 (TM1), TM6, TM7, the third extracellular loop and the intracellular carboxy-terminus of chimera A with the corresponding glucagon receptor segments re-established the ability to distinguish GLP-1(7-20)/glucagon(15-29) from glucagon. Corroborant results were obtained with the opposite chimeric peptide glucagon(1-14)/GLP-1(21-37). (3) The results suggest that the glucagon and GLP-1 receptor amino-terminal extracellular domains determine specificity for the divergent residues in the glucagon and GLP-1 carboxy-terminals respectively. The GLP-1 receptor core domain is not a critical determinant of glucagon/GLP-1 selectivity. Conversely, the glucagon receptor core domain contains two or more sub-segments which strongly determine specificity for divergent residues in the glucagon amino-terminus.

VIP as a trophic factor in the CNS and cancer cells

The effects of vasoactive intestinal peptide (VIP) on the proliferation of central nervous system (CNS) and cancer cells were investigated. VIP has important actions during CNS development. During neurogenesis, VIP stimulates the proliferation and differentiation of brain neurons. Addition of VIP to embryonic mouse spinal cord cultures increases neuronal survival and activity dependent neurotrophic factor (ADNF) secretion from astroglial cells. VIP is an integrative regulator of brain growth and development during neurogenesis and embryogenesis. Also, VIP causes increased proliferation of human breast and lung cancer cells in vitro. VIP binds with high affinity to cancer cells, elevates the cAMP and increases gene expression of c-fos, c-jun, c-myc and vascular endothelial cell growth factor. The effects of VIP on cancer cells are reversed by VIPhybrid, a synthetic VPAC(1) receptor antagonist. VIPhyb inhibits the basal growth of lung cancer cells in vitro and tumors in vivo and potentiates the ability of chemotherapeutic drugs to kill cancer cells. Due to the high density of VPAC(1) receptors in cancer cells, VIP has been radiolabeled with 123I, 18F and 99mTc to image tumors. It remains to be determined if radiolabeled VIP analogs will be useful agents for early detection of cancer in patients.

Molecular pharmacology and structure of VPAC Receptors for VIP and PACAP

VIP and PACAP are two prominent neuropeptides which share two common G protein-coupled receptors VPAC1 and VPAC2 while PACAP has an additional specific receptor PAC1. This paper reviews the present knowledge regarding three aspects of VPAC receptors including: (i). receptor specificity towards natural VIP-related peptides and pharmacology of synthetic agonists or antagonists; (ii). receptor signaling; (iii). molecular basis of ligand-receptor interaction as determined by site-directed mutagenesis, construction of receptor chimeras and structural modeling.

Human VPAC1 receptor selectivity filter. Identification of a critical domain for restricting secretin binding

The human VPAC1 receptor for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating peptide (PACAP) belongs to the class II family of G protein coupled receptors with seven transmembrane segments. It recognizes several VIP-related peptides and displays a very low affinity for secretin despite >70% homology between VIP and secretin. Conversely, the human secretin receptor has high affinity for secretin but low affinity for VIP. We took advantage of this reversed selectivity to identify a domain of the VPAC1 receptor responsible for selectivity toward secretin by constructing human VPAC1-secretin receptor chimeras. A first set of chimeras consisted of exchanging the entire N-terminal ectodomain or large parts of this domain. They were constructed by overlap PCR, transfected in COS-7 cells, and their ligand selectivity, expressed as the ratio of EC(50) for secretin/EC(50) for VIP (referred to as S/V), in stimulating cAMP production was measured. Two very informative chimeras respectively referred to as S144V and S123V were obtained by replacing the entire ectodomain or only the first 123 amino acids of the VPAC1 receptor by the corresponding sequences of the secretin receptor. Whereas S144V no longer discriminated between VIP and secretin (S/V = 1.2), S123V discriminated between the two peptides (S/V = 300) in the same manner as the wild-type VPAC1 receptor. The motif responsible for discrimination was determined by introducing small blocks or individual amino acids of secretin receptor in the 123-144 sequence of the S123V chimera. The data obtained from 14 new chimeras sustained that two nonadjacent pairs of amino acids, Gln(135) Thr(136) and Gly(140) Ser(141) in the C-terminal end of the N-terminal VPAC1 receptor ectodomain constitute a selective filter that strongly restricts access of secretin to the VPAC1 receptor.

Expression and fine mapping of murine vasoactive intestinal peptide receptor 1

Vasoactive intestinal peptide (VIP) plays multiple roles in the nervous, endocrine, and immune systems as a neurotransmitter, a hormone, and a cytokine. VIP is widely distributed in neurons of the central and peripheral nervous systems (CNS/PNS), and recently has been found to be an important neuroprotective agent. VIP actions are mediated through specific G protein-coupled receptors. We have cloned the cDNA of VIP receptor subtype 1 (VIPR1 or VPAC1) and have demonstrated the quantitative expression profile in mice. Fluorometric real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that VPAC1 is expressed in all tissues examined. Expression was highest in the small intestine and colon followed by the liver and brain. The high level of VPAC1 expression in forebrain and cerebellum suggests that VPAC1 may mediate the neuroprotective effect of VIP. We have refined the chromosomal localization of the mouse, rat, and human VPAC1 genes. This fine mapping of the VPAC1 gene extends the respective regions of synteny between the distal region of mouse chromosome 9, rat chromosome 8q32, and human chromosome 3p21.33-p21.31. Thus, VPAC, constitutes a functional-positional candidate for the tumor-suppressor function mapped to human 3p22-p21 where loss-of-heterozygosity is observed in small-cell lung carcinoma (SCLC) cell lines and primary tumors. Availability of the cDNA sequences for mouse VPAC1 will facilitate the generation of VPAC1 null mutant animals. Such studies will ultimately enhance our understanding of the role of VIP in the nervous system.

Transglutaminase-mediated polyamination of vasoactive intestinal peptide (VIP) Gln16 residue modulates VIP/PACAP receptor activity

Previous data showing an increase of receptor binding activity of [R16]VIP, a vasoactive intestinal peptide (VIP) structural analogue containing arginine at the position 16 of its amino acid sequence, have pointed out the importance of a positive charge at this site. Here, the functional characterization of three VIP polyaminated adducts (VIPDap, VIPSpd, and VIPSpm), obtained by a transglutaminase-catalysed reaction between the VIP Gln16 residue and 1,3-diaminopropane (Dap), spermidine (Spd), or spermine (Spm), is reported. Appropriate binding assays and adenylate cyclase enzymatic determinations have shown that these VIP adducts act as structural VIP agonists, both in vitro and in vivo. In particular, their IC50 and EC50 values of human and rat VIP/pituitary adenylate cyclase activating peptide (PACAP)1 and VIP/PACAP2 receptors indicate that VIPDap is a VIP agonist, with an affinity and a potency higher than that of VIP, while VIPSpd and VIPSpm are also agonists but with affinities lower than that of VIP. These findings suggest that the difference in adduct agonist activity reflects the differences in the positive charge and carbon chain length of the polyamine covalently linked with the VIP Gln16 residue. In addition, the data obtained strongly suggest that the length of polyamine carbon chain could be critical for the interaction of the agonist with its receptor, even though possible hydrophobic interaction cannot be ruled out. In vivo experiments on murine J774 macrophage cell cultures have shown the ability of these compounds to stimulate the inducible nitric oxide synthase activity at the transcriptional level.

(N-stearyl, norleucine17)VIPhybrid is a broad spectrum vasoactive intestinal peptide receptor antagonist

The effects of a (N-stearyl, Norleucine17) vasoactive intestinal peptide hybrid ((SN)VIPhybrid) on cells stably transfected with VPAC,, VPAC2, or PAC1 receptors were investigated. (SN)VIPhybrid inhibited specific 125I-VIP binding to membranes derived from CHO cells transfected with VPAC, or VPAC2 receptors with high affinity (IC50 = 30 and 50 nM). (SN)VIPhyb inhibited specific 125I-PACAP-27 binding to membranes derived from NIH/3T3 cells transfected with PAC1 receptors with high affinity (IC50 = 65 nM). PACAP-27 caused cAMP elevation in NIH/3T3 cells transfected with PAC1 receptors and the increase cAMP caused by pituitary adenylated cyclase (PACAP) was inhibited by (SN)VIPhyb. Also, the increase in cAMP caused by VIP using CHO cells transfected with VPAC1 or VPAC2 receptors was antagonized by (SN)VIPhyb. These results indicate that (SN)VIPhyb is an antagonist for VPAC1, VPAC2, and PAC1 receptors.

Vasoactive intestinal polypeptide as mediator of asthma

Vasoactive intestinal polypeptide (VIP) is one of the most abundant, biologically active peptides found in the human lung. VIP is a likely neurotransmitter or neuromodulator of the inhibitory non-adrenergic non-cholinergic airway nervous system and influences many aspects of pulmonary biology. In human airways VIP-immunoreactive nerve fibres are present in the tracheobronchial airway smooth muscle layer, the walls of pulmonary and bronchial vessels and around submucosal glands. Next to its prominent bronchodilatory effects, VIP potently relaxes pulmonary vessels. The precise role of VIP in the pathogenesis of asthma is still uncertain. Although a therapy using the strong bronchodilatory effects of VIP would offer potential benefits, the rapid inactivation of the peptide by airway peptidases has prevented effective VIP-based drugs so far and non-peptide VIP-agonists did not reach clinical use.

Expression and fine mapping of murine vasoactive intestinal peptide receptor 1

Vasoactive intestinal peptide (VIP) plays multiple roles in the nervous, endocrine, and immune systems as a neurotransmitter, a hormone, and a cytokine. VIP is widely distributed in neurons of the central and peripheral nervous systems (CNS/PNS), and recently has been found to be an important neuroprotective agent. VIP actions are mediated through specific G protein-coupled receptors. We have cloned the cDNA of VIP receptor subtype 1 (VIPR1 or VPAC1) and have demonstrated the quantitative expression profile in mice. Fluorometric real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that VPAC1 is expressed in all tissues examined. Expression was highest in the small intestine and colon followed by the liver and brain. The high level of VPAC1 expression in forebrain and cerebellum suggests that VPAC1 may mediate the neuroprotective effect of VIP. We have refined the chromosomal localization of the mouse, rat, and human VPAC1 genes. This fine mapping of the VPAC1 gene extends the respective regions of synteny between the distal region of mouse chromosome 9, rat chromosome 8q32, and human chromosome 3p21.33-p21.31. Thus, VPAC, constitutes a functional-positional candidate for the tumor-suppressor function mapped to human 3p22-p21 where loss-of-heterozygosity is observed in small-cell lung carcinoma (SCLC) cell lines and primary tumors. Availability of the cDNA sequences for mouse VPAC1 will facilitate the generation of VPAC1 null mutant animals. Such studies will ultimately enhance our understanding of the role of VIP in the nervous system.

Proline residue 280 in the second extracellular loop (EC2) of the VPAC2 receptor is essential for the receptor structure

Inspection of the amino acid sequence of the human VPAC1 and the VPAC2 receptors after alignment of the conserved residues indicates that the second extracellular loop (EC2) is one amino acid shorter in the VPAC1 receptor due to the lack of a proline residue in position 294. We hypothesized that this could be of importance for receptor structure and/or for ligand recognition. Insertion by directed mutagenesis of a proline in that position (<Pro>294 VPAC1) had little consequence on the binding of several agonists but reduced the affinity for the VPAC1 antagonist. Coupling of the <Pro>294 VPAC1 receptor to adenylate cyclase was improved, as demonstrated by an increased affinity for VIP and other agonists, and by a shift of the VPAC1 antagonist to partial agonist behavior. Deletion of the proline 280 (DeltaPro280 VPAC2) in the VPAC2 receptor markedly reduced the apparent affinity for all the agonists tested. Replacement of the proline by a glycine residue had a smaller effect on the ligands affinities. The proline residue in the VPAC2 receptor EC2 is thus essential for the receptor structure, and the EC2 domain is involved in ligand recognition and receptor functionality.

VIP receptor antagonists and chemotherapeutic drugs inhibit the growth of breast cancer cells

The effects of vasoactive intestinal peptide (VIP) antagonists on breast cancer cells were investigated. (N-stearyl, norleucine17)VIP hybrid ((SN)VIPhyb) inhibited specific 125I-VIP binding to MCF7, SKBR3, T47D ZR75-1 and MDA-MB231 cells with high affinity (IC50 values of 0.03-0.06 microM). (SN)VIPhyb, 1 microM, inhibited the ability of 10 nM VIP to cause elevation of cAMP and to increase c-fos mRNA. Micromolar concentrations of (SN)VIPhyb inhibited the proliferation of MDA-MB231 or MCF7 cells using a MTT and clonogenic assay. Using a MTT assay, (SN)VIPhyb enhanced the ability of taxol and doxorubicin to inhibit breast cancer growth. Using nude mice bearing MDA-MB231 xenografts, VIPhyb potentiated the ability of taxol to inhibit proliferation. The results indicate that VIP receptor antagonists increase the ability of chemotherapeutic drugs to kill breast cancer cells.

The human VPAC1 receptor: three-dimensional model and mutagenesis of the N-terminal domain

The human VPAC(1) receptor for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating peptide belongs to the class II family of G-protein-coupled receptors with seven transmembrane segments. Like for all class II receptors, the extracellular N-terminal domain of the human VPAC(1) receptor plays a predominant role in peptide ligand recognition. To determine the three-dimensional structure of this N-terminal domain (residues 1-144), the Protein Data Bank (PDB) was screened for a homologous protein. A subdomain of yeast lipase B was found to have 27% sequence identity and 50% sequence homology with the N-terminal domain (8) of the VPAC(1) receptor together with a good alignment of the hydrophobic clusters. A model of the N-terminal domain of VPAC(1) receptor was thus constructed by homology. It indicated the presence of a putative signal sequence in the N-terminal extremity. Moreover, residues (Glu(36), Trp(67), Asp(68), Trp(73), and Gly(109)) which were shown to be crucial for VIP binding are gathered around a groove that is essentially negatively charged. New putatively important residues for VIP binding were suggested from the model analysis. Site-directed mutagenesis and stable transfection of mutants in CHO cells indicated that Pro(74), Pro(87), Phe(90), and Trp(110) are indeed important for VIP binding and activation of adenylyl cyclase activation. Combination of molecular modeling and directed mutagenesis provided the first partial three-dimensional structure of a VIP-binding domain, constituted of an electronegative groove with an outspanning tryptophan shell at one end, in the N-terminal extracellular region of the human VPAC(1) receptor.

Development of selective agonists and antagonists for the human vasoactive intestinal polypeptide VPAC(2) receptor

Ro 25-1553 is a cyclic VIP derivative with a high affinity for the VPAC(2) receptor subtype. Our goal was to identify the modifications that support its selectivity for VPAC(2) receptors, and to develop a VIP or Ro 25-1553 analog behaving as a high affinity, VPAC(2) selective antagonist. The selectivity of Ro 25-1553 for the human receptor was supported mainly by the acetylation of the amino-terminus, by the introduction of a lysine residue in position 12, and by the carboxyl-terminal extension. The lactam bridge created between positions 21 and 25 contributed to the affinity of the compound for the VIP receptors but participated only marginally to its selectivity. Deletion of the first five aminoacid residues led to a low affinity antagonist with a low selectivity. Introduction of a D-Phe residue in position 2 reduced the affinity, the selectivity and the intrinsic activity, the compound being a partial agonist. Myristoylation of the amino-terminus of [K(12)]VIP(1-26) extended carboxyl-terminally with the -K-K-G-G-T sequence of Ro 25-1553 led to a high affinity, selective VPAC(2) receptor antagonist. This molecule represents the first selective human VPAC(2) receptor antagonist described to date.

Tryptophan 67 in the human VPAC(1) receptor: crucial role for VIP binding

The human receptor subtype for VIP and PACAP, referred to as VPAC(1) receptor, has a large N-terminal extracellular domain which is critical for VIP binding. We further investigated this domain by mutating 12 amino acid residues which could participate in the formation of a tight bend (W67) or a coiled coil motif. They were changed to alanine (A) and the cDNAs were transiently transfected into Cos cells. All mutants but W67A exhibited K(d) values similar to that of the wild-type receptor. For the W67A mutant, no specific (125)I-VIP binding could be observed. Mutants at the W67 site were further characterized after stable transfection of epitope-tagged VPAC(1) receptor-GFP fusion proteins into CHO cells. W67A, W67E, W67H, and W67K mutants neither bound VIP nor mediated adenylyl cyclase activation by VIP. The W67F mutant mediated stimulation of adenylyl cyclase only at high VIP concentrations. Microscopic analysis and antibody binding experiments showed that all mutants were similarly expressed at the cell surface of CHO cells. Therefore tryptophan 67 in the human VPAC(1) receptor plays a crucial role in VIP binding due, in part, to its aromatic moiety.

Inhibition of expression of the type I G protein-coupled receptor for vasoactive intestinal peptide (VPAC1) by hammerhead ribozymes

Vasoactive intestinal peptide (VIP) is a neuromediator expressed widely in the nervous, gastrointestinal, respiratory, and immune systems. Two G protein-coupled receptors (GPCRs), designated VPAC1 and VPAC2, bind VIP with high affinity and transduce increases in [cyclic AMP](i) and [Ca(2+)](i). As there are no potent VPAC1- or VPAC2-selective antagonists, a hammerhead ribozyme (Rz) strategy capable of in vivo application was adopted to inactivate individual domains of VPAC1. Three Rzs were designed to cleave mRNA encoding the amino terminus, the third intracellular loop, and the cytoplasmic tail of human VPAC1 and were introduced by transfection into HEK-293 cells expressing recombinant human VPAC1. Each Rz specifically degraded VPAC1 mRNA and down-regulated VPAC1 protein and VIP-binding activity, as assessed by ribonuclease protection assays, Western blots, and binding of (125)I-VIP. Rz-mediated down-regulation of VPAC1 was associated with up to 75% suppression of VIP signaling of increases in [cyclic AMP](i) and [IP3](i), and of cyclic AMP response element-luciferase reports. The Rz specific for the amino terminus inhibited VPAC1 expression and signaling to the greatest extent. VIP-evoked cellular responses thus appear to be proportional to the level of VPAC1 expression. Specific Rzs may be powerful tools for manipulating tissue-specific contributions of GPCRs in vitro and in vivo.

Identification of key residues for interaction of vasoactive intestinal peptide with human VPAC1 and VPAC2 receptors and development of a highly selective VPAC1 receptor agonist. Alanine scanning and molecular modeling of the peptide

The widespread neuropeptide vasoactive intestinal peptide (VIP) has two receptors VPAC(1) and VPAC(2). Solid-phase syntheses of VIP analogs in which each amino acid has been changed to alanine (Ala scan) or glycine was achieved and each analog was tested for: (i) three-dimensional structure by ab initio molecular modeling; (ii) ability to inhibit (125)I-VIP binding (K(i)) and to stimulate adenylyl cyclase activity (EC(50)) in membranes from cell clones stably expressing human recombinant VPAC(1) or VPAC(2) receptor. The data show that substituting residues at 14 positions out of 28 in VIP resulted in a >10-fold increase of K(i) or EC(50) at the VPAC(1) receptor. Modeling of the three-dimensional structure of native VIP (central alpha-helice from Val(5) to Asn(24) with random coiled N and C terminus) and analogs shows that substitutions of His(1), Val(5), Arg(14), Lys(15), Lys(21), Leu(23), and Ile(26) decreased biological activity without altering the predicted structure, supporting that those residues directly interact with VPAC(1) receptor. The interaction of the analogs with human VPAC(2) receptor is similar to that observed with VPAC(1) receptor, with three remarkable exceptions: substitution of Thr(11) and Asn(28) by alanine increased K(i) for binding to VPAC(2) receptor; substitution of Tyr(22) by alanine increased EC(50) for stimulating adenylyl cyclase activity through interaction with the VPAC(2) receptor. By combining 3 mutations at positions 11, 22, and 28, we developed the [Ala(11,22,28)]VIP analog which constitutes the first highly selective (>1,000-fold) human VPAC(1) receptor agonist derived from VIP ever described.

Potentiation of the inhibitory effect of growth hormone-releasing hormone antagonists on PC-3 human prostate cancer by bombesin antagonists indicative of interference with both IGF and EGF pathways

BACKGROUND

In view of the involvement of various neuropeptides and growth factors in the progression of androgen-independent prostate cancer, we investigated the effects of antagonists of growth hormone-releasing hormone (GHRH) alone or in combination with an antagonist of bombesin/gastrin-releasing peptide (BN/GRP) on PC-3 human prostate cancers.

METHODS

Nude mice implanted with PC-3 tumors received GHRH antagonists MZ-5-156 or JV-1-38, each at 20 microgram/day s.c. In experiment 2, treatment consisted of daily injections of JV-1-38 (20 microgram), BN/GRP antagonist RC-3940-II (10 microgram), or a combination of JV-1-38 and RC-3940-II. Serum IGF-I levels, expression of mRNA for IGF-II, and characteristics of BN/GRP and EGF receptors in tumor tissue were investigated.

RESULTS

JV-1-38 induced a greater inhibition of tumor growth and suppression of IGF-II mRNA than MZ-5-156, both compounds causing a similar decrease in serum IGF-I. In experiment 2, JV-1-38 and RC-3940-II produced a comparable reduction in tumor volume (65% and 61%, respectively), but a combination of both antagonists augmented tumor inhibition to 75%. Combined treatment with JV-1-38 and RC-3940-II also led to a greater suppression of IGF-II mRNA (92%), as compared with JV-1-38 (72%) or RC-3940-II (77%). Serum IGF-I concentration was lowered only in mice treated with JV-1-38, while the downregulation of BN/GRP and EGF receptors was specific for groups receiving RC-3940-II.

CONCLUSIONS

The inhibitory effects of GHRH antagonists on PC-3 human androgen-independent prostate cancer can be potentiated by concomitant use of BN/GRP antagonists. The combination of both types of analogs apparently interferes with both IGF and bombesin/EGF pathways, and might be clinically useful for the management of androgen-independent prostate cancer.

Evidence that the lizard helospectin peptides are O-glycosylated

Six forms of helospectin (a vasoactive intestinal peptide analogue) were purified from the venom of the Heloderma horridum lizard. Their identification was performed by combining sequencing by automated Edman degradation and electrospray mass spectrometry analysis on the complete peptides and their tryptic fragments. The products resulting from the action of an O-glycosidase were also analysed. Two forms were identified as the previously named Hs1 and Hs2 of 38 and 37 amino-acid residues, respectively. Two forms corresponded to Hs1 and Hs2 O-glycosylated by a N-acetylhexosamine-hexose motif attached to the Ser32 residue. Two other forms were not completely characterized but might correspond to the O-glycosylated forms bearing a phosphate or a sulfate group. The glycosylation did not affect the capacity of the helospectins to recognize and to activate the human and the rat VPAC1 and VPAC2 receptors.

Characterization of a novel VPAC(1) selective agonist and identification of the receptor domains implicated in the carboxyl-terminal peptide recognition

Vasoactive Intestinal Polypeptide (VIP) interacts with a high affinity to two subclasses of G protein coupled receptors named VPAC(1) and VPAC(2), and has a 3 - 10 fold preference for VPAC(1) over VPAC(2) receptors. Selective ligands for each receptor subclass were recently described. [R(16)]-PACAP (1 - 23) and [L(22)]-VIP are two selective VPAC(1) agonists. Chimaeric human VPAC(2)-VPAC(1) recombinant receptors expressed in CHO cells were used to identify the receptor domains implicated in these two selective ligands recognition. The VPAC(2) preference for [R(16)]-PACAP (1 - 27) over [R(16)]-PACAP (1 - 23) did not require the receptor's NH(2)-terminus domain but involved the whole transmembrane domain. In contrast, the selectivity of [L(22)]-VIP depended only on the presence of the NH(2) terminus and EC(2) domains of the VPAC(1) receptor. The present data support the idea that in the GPCR-B family of receptors the different selective ligands require different domains for their selectivity, and that the peptides carboxyl terminal sequence (amino acids 24 - 27) folds back on the transmembrane receptor domain, close to the peptides, aminoterminus.

The human vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor 1 (VPAC1) promoter: characterization and role in receptor expression during enterocytic differentiation of the colon cancer cell line Caco-2Cl.20

The basic organization of the human vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor (VPAC) 1 promoter was investigated after cloning the 5'-flanking region (1.4 kb) of the VPAC1 gene from a human genomic library. Subsequent functional analysis of various deletions of the 5'-flanking sequence, subcloned upstream of a luciferase reporter gene, was carried out in HT-29 cells. The minimal promoter region identified encompasses the -205/+76 sequence and contains a crucial CCAAT box (-182/-178) and a GC-rich sequence. Moreover a region (-1348/-933) containing a silencer element was identified. We previously showed that the expression of the VPAC1 receptor binding site is strictly dependent upon the enterocytic differentiation of human colon cancer Caco-2 cells [Laburthe, Rousset, Rouyer-Fessard, Couvineau, Chantret, Chevalier and Zweibaum (1987) J. Biol. Chem. 262, 10180-10184]. In the present study we show that VPAC1 mRNA increases dramatically when Caco-2Cl.20 cells differentiate, as measured by RNase protection assays and reverse transcriptase-PCR. A single transcript species of 3 kb is detected in differentiated cells by Northern-blot analysis. Accumulation of VPAC1 receptor mRNA is due to a 5-fold increase of transcription rate (run-on assay) without a change in mRNA half-life (9 h). Stable transfections of various constructs in Caco-2Cl.20 cells and subsequent analysis of reporter gene expression, during the enterocytic differentiation process over 25 days of culture, further indicated that the -254/+76 5'-flanking sequence is endowed with the regulatory element(s) necessary for transcriptional regulation of VPAC1 during differentiation. Altogether, these observations provide the first characterization of the basic organization of the human VPAC1 gene promoter and unravel the crucial role of a short promoter sequence in the strict transcriptional control of VPAC1 expression during differentiation of human colon cancer Caco-2 cells.

(N-stearyl, norleucine17) VIP hybrid inhibits the growth of pancreatic cancer cell lines

The effects vasoactive intestinal peptide (VIP) antagonists were investigated on pancreatic cancer cell lines. (N-Stearyl, Norleucine17) VIP hybrid ((SN)VIPhyb) inhibited 125I-VIP binding to human Capan-2 cells with an IC50 value of 0.01 microM whereas VIP hybrid had an IC50 value of 0.2 microM. By RT-PCR and Northern blot, VPAC1 receptor mRNA was detected in CAPAN-2 cells. One microM (SN)VIPhyb and 10 microM VIPhyb inhibited the ability of 30 nM VIP to elevate cyclic AMP and increase c-fos mRNA. (SN)VIPhyb, 1 microM inhibited the clonal growth of CAPAN-2 cells in vitro. In vivo, (SN)VIPhyb (10 microg/day s.c.) inhibited CAPAN-2 xenograft growth in nude mice. These results indicate that (SN)VIPhyb is a pancreatic cancer VPAC receptor antagonist.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

The hydrophobic residues phenylalanine 184 and leucine 187 in the type-1 parathyroid hormone (PTH) receptor functionally interact with the amino-terminal portion of PTH-(1-34)

Recent mutagenesis and cross-linking studies suggest that three regions of the PTH-1 receptor play important roles in ligand interaction: (i) the extreme NH(2)-terminal region, (ii) the juxtamembrane base of the amino-terminal extracellular domain, and (iii) the third extracellular loop. In this report, we analyzed the second of these segments in the rat PTH-1 receptor (residues 182-190) and its role in functional interaction with short PTH fragment analogs. Twenty-eight singly substituted PTH-1 receptors were transiently transfected into COS-7 cells and shown to be fully expressed by surface antibody binding analysis. Alanine-scanning analysis identified Phe(184), Arg(186), Leu(187), and Ile(190) as important determinants of maximum binding of (125)I-labeled bovine PTH-(1-34) and (125)I-labeled bovine PTH-(3-34) and determinants of responsiveness to the NH(2)-terminal analog, PTH-(1-14) in cAMP stimulation assays. Alanine mutations at these four sites augmented the ability of the COOH-terminal peptide [Glu(22), Trp(23)]PTHrP-(15-36) to inhibit the cAMP response induced by PTH-(1-34). At Phe(184) and Leu(187), hydrophobic substitutions (e.g. Ile, Met, or Leu) preserved PTH-(1-34)-mediated cAMP signaling potency, whereas hydrophilic substitutions (e.g. Asp, Glu, Lys, or Arg) weakened this response by 20-fold or more, as compared with the unsubstituted receptor's response. The results suggest that hydrophobicity at positions occupied by Phe(184) and Leu(187) in the PTH-1 receptor plays an important role in determining functional interaction with the 3-14 portion of PTH.

Domains determining agonist selectivity in chimaeric VIP2 (VPAC2)/PACAP (PAC1) receptors

1 The VPAC2 and PAC1 receptors are closely related members of the Group II G protein-coupled receptor family. At the VPAC2 receptor, VIP is equipotent to PACAP-38 in stimulating cyclic AMP production, whereas at the PAC1 receptor PACAP-38 is many fold more potent than VIP. In this study, domains which confer this selectivity were investigated by constructing four chimaeric receptors in which segments of the VPAC2 receptor were exchanged with the corresponding segment from the PAC1 receptor. 2 When expressed in COS 7 cells all the chimaeric receptors bound the common ligand [125I]PACAP-27 and produced cyclic AMP in response to agonists. 3 Relative selectivity for agonists was determined primarily by the amino terminal extracellular domain of the PAC1 receptor and the VPAC2 receptor. The interchange of other domains had little effect on the potency of PACAP-38 or PACAP-27. 4 For chimaeric constructs with a PAC1 receptor amino terminal domain, the substitution of increasing portions of the VPAC2 receptor decreased the potency of VIP yet increased that of helodermin. 5 This suggests that the interaction of VIP/helodermin but not PACAP with the PAC1 receptor may be influenced (and differentially so) by additional receptor domains.

Vasoactive intestinal polypeptide VPAC1 and VPAC2 receptor chimeras identify domains responsible for the specificity of ligand binding and activation

In order to identify the receptor domains responsible for the VPAC1 selectivity of the VIP1 agonist, [Lys15, Arg16, Leu27] VIP (1-7)/GRF (8-27) and VIP1 antagonist, Ac His1 [D-Phe2, Lys15, Arg16, Leu27] VIP (3-7)/GRF (8-27), we evaluated their binding and functional properties on chimeric VPAC1/VPAC2 receptors. Our results suggest that the N-terminal extracellular domain is responsible for the selectivity of the VIP1 antagonist. Selective recognition of the VIP1 agonist was supported by a larger receptor area: in addition to the N-terminal domain, the first extracellular loop, as well as additional determinants in the distal part of the VPAC1 receptor were involved. Furthermore, these additional domains were critical for an efficient receptor activation, as replacement of EC1 in VPAC1 by its counter part in the VPAC2 receptor markedly reduced the maximal response.

The ligand-selective domains of corticotropin-releasing factor type 1 and type 2 receptor reside in different extracellular domains: generation of chimeric receptors with a novel ligand-selective profile

The nonselective human corticotropin-releasing factor (hCRF) receptor 1 (hCRFR1) and the ligand-selective Xenopus CRFR1 (xCRFR1), xCRFR2, and hCRFR2alpha were compared. To understand the interactions of hCRF, ovine CRF (oCRF), rat urocortin (rUcn), and sauvagine, ligands with different affinities for type 1 and type 2 CRFRs, chimeric and mutant receptors of hCRFR1, xCRFR1, hCRFR2alpha, and xCRFR2 were constructed. In cyclic AMP stimulation and CRF-binding assays, it was established that different extracellular regions of CRFR1 and CRFR2 conferred their ligand selectivities. The ligand selectivity of xCRFR1 resided in five N-terminal amino acids, whereas the N-terminus of both CRFR2 proteins did not contribute to their ligand selectivities. Chimeric receptors in which the first extracellular domain of hCRFR1 replaced that of hCRFR2alpha or xCRFR2 showed a similar pharmacological profile to the two parental CRFR2 molecules. Chimeric receptors carrying the N-terminal domain of xCRFR1 linked to hCRFR2alpha or xCRFR2 displayed a novel pharmacological profile. hCRF, rUcn, and sauvagine were bound with high affinity, whereas oCRF was bound with low affinity. Furthermore, when three or five residues of xCRFR1 (Gln76, Gly81, Val83, His88, Leu89; or Gln76, Gly81, Val83) were introduced into receptor chimeras carrying the N-terminus of hCRFR1 linked to xCRFR2, the same novel pharmacology was observed. These data indicate a compensation mechanism of two differentially selecting regions located in different domains of both xCRFR1 and CRFR2.

A cloned frog vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating polypeptide receptor exhibits pharmacological and tissue distribution characteristics of both VPAC1 and VPAC2 receptors in mammals

Three receptor subtypes for the neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) have been identified in mammals: the PAC1 receptor (PAC1-R) which is selectively activated by PACAP, and two VPAC receptors (VPAC1-R and VPAC2-R), which are equally stimulated by PACAP and VIP. The structures of PACAP and VIP have been well conserved during evolution, but little is known about VIP/PACAP receptors in nonmammalian species. An amphibian VIP/PACAP receptor complementary DNA (cDNA) has been cloned and characterized from a frog (Rana ridibunda) pituitary cDNA library. The predicted protein contains seven putative transmembrane domains and exhibits the highest sequence identity (65%) with the human VPAC1-R. The cloned cDNA was transiently expressed in LLC-PK1 cells, and its pharmacological profile was determined in comparison with the human VPAC1-R. Both PACAP and VIP stimulated cAMP accumulation through the cloned receptor with an EC50 of about 30 nM. In contrast, secretin, at concentrations that stimulate the human VPAC1-R, had no effect on cAMP production. RT-PCR analysis revealed the widespread distribution of this frog VIP/PACAP receptor in peripheral tissues. In situ hybridization histochemistry using a complementary RNA probe showed that the receptor gene is highly expressed in several hypothalamic and thalamic nuclei and to a lesser extent in the pallium and striatum. In the pituitary, the highest messenger RNA levels were detected in the distal lobe. Taken together, these data show that the cloned frog receptor shares several common features with both the VPAC1-R and VPAC2-R of mammals; the frog receptor exhibits the highest sequence identity with mammalian VPAC1-R, but the lack of effect of secretin and the brain distribution of the receptor are reminiscent of the characteristics of the mammalian VPAC2-R. The sequence of the frog receptor should thus prove useful to decipher the structure-activity relationships of the VIP/PACAP receptor family.

Functional Expression of Receptors for Calcitonin Gene-Related Peptide, Calcitonin, and Vasoactive Intestinal Peptide in the Human Thymus and Thymomas from Myasthenia Gravis Patients

The molecular and functional expression of serpentine membrane receptors for vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), and calcitonin (CT) were characterized in human thymus and thymomas from myasthenia gravis (MG) patients and thymic epithelial cells either in primary culture (PTEC) or transformed by the siman virus 40 large T (SV40LT) oncogene (LT-TEC). Using RT-PCR combined with Southern analysis, we identified the PCR products corresponding to the receptor (-R) transcripts for VIP, CGRP, and CT in thymus from control subjects and MG patients with either hyperplasia or thymoma. Similar expressions of the VIP- and CGRP-R transcripts were observed in PTEC, whereas the CT-R message was not detected. In LT-TEC, the signals for VIP-R, CGRP-R, and CT-R transcripts were seen with a lower intensity than those in control and MG thymus. In agreement with our molecular analysis, 1) VIP was the most potent peptide among VIP-related peptides (VIP &gt; PACAP &gt; PHM &gt; PHV) to stimulate cAMP production through specific type 1 VIP receptors in both PTEC and LT-TEC; 2) cAMP generation was induced by CGRP in PTEC and by CT in LT-TEC; 3) in frozen thymic sections and by flow cytometry, type 1 VIP-R, CGRP-R, and CT-R were localized in epithelial cells; and 4) in parallel, the transcription of the acetylcholine receptor α subunit (the main autoantigen in MG) was induced by CGRP and CT in PTEC and LT-TEC, respectively. Our data suggest that the neuroendocrine peptides VIP, CGRP, and CT may exert functional roles during MG and malignant transformation of the human thymus.