The vasoactive intestinal peptide receptor on intact human colonic adenocarcinoma cells (HT29-D4). Evidence for its glycoprotein nature

Functional reconstitution of membrane glycoproteins into lipid vesicles using lectin precipitation. Application to the VIP receptor

We studied the interaction of n-octyl-beta-d-glucopyranoside-solubilized VIP receptors (VIPR) with wheat germ agglutinin and found that the addition of the lectin to the detergent extract led to the formation of aggregates that could be pelleted by high speed centrifugation. Resuspension of the pellet in the presence of the competing trisaccharide, N,N', N"-triacetylchitotriose (TAC), dissociated the lectin from the complex without altering the precipitability of VIPR. The final pellet (referred to as TAC pellet) contained an average of 12% of total protein and 96% of total VIP binding activity with a typical rank order of potency for VIP-related peptides. Lipid analysis and electron microscopic examination indicated that the precipitated material was composed of lipid vesicles. VIPR molecules were shown to be integrally inserted in the liposomes because they could not be dissociated from the vesicles at pH 11 or with high salt concentration. By comparing the liposome-associated VIP binding activity in the presence and absence of detergent and by showing accessibility of VIPR to PNGase F, it was concluded that VIP binding sites were not simply trapped within the reconstituted vesicles but likely exposed at the external surface of the liposomes.

Mutagenesis of N-glycosylation sites in the human vasoactive intestinal peptide 1 receptor. Evidence that asparagine 58 or 69 is crucial for correct delivery of the receptor to plasma membrane

The functional role of N-linked carbohydrates in the human vasoactive intestinal peptide (VIP) 1 receptor was investigated by site-directed mutagenesis (Asn-->Thr) of the four consensus N-glycosylation sites on Asn58, Asn69, Asn100 (N-terminal extracellular domain) and Asn293 (second extracellular loop). Mutated receptors were investigated after transient expression in Cos-7 cells, by ligand binding assay, affinity cross-linking, western blotting, and confocal laser microscopy of epitope-tagged receptor proteins. Mutations of each consensus site revealed that Asn58, Asn69, and Asn100 were occupied by a 9-kDa N-linked carbohydrate whereas Asn293 was not used for glycosylation. Each mutated receptor was expressed (western blot) and delivered at the plasma membrane (confocal microscopy) of Cos-7 cells. They displayed a dissociation constant similar to that of the wild-type receptor, i.e., 0.5-1 nM. In contrast, no VIP binding to Cos-7 cells could be observed with the mutant devoid of consensus N-glycosylation sites due to a strict sequestration of this mutant in the perinuclear endoplasmic reticulum. However, when solubilized with a zwitterionic detergent, this mutant bound [125I]VIP specifically, indicating that it retained intrinsic binding activity. The construction of other mutants in which three out of four N-glycosylation sites were altered, demonstrated that N-glycosylation at either Asn58 or Asn69 is necessary and sufficient to ensure correct delivery of the receptor to the plasma membrane. Further pharmacological studies involving incubation of Cos-7 cells with castanospermine or deoxymannojirimycin immediately after transfection of mutated cDNAs encoding receptors with a single glycosylation site at Asn58 or Asn69 suggested that carbohydrate at Asn58 was involved in a calnexin-dependent folding process of the receptor whereas carbohydrate at Asn69 was not. These studies highlight the functional importance of the N-glycosylation of the human VIP 1 receptor which belongs to a new subfamily of seven membrane-spanning receptors.

Receptors for gut regulatory peptides

Receptors for regulatory peptides (hormones or neurotransmitters) play a pivotal role in the ability of cells to taste the rich neuroendocrine environment of the gut. Recognition of low concentration of peptides with a high specificity and translation of the peptide-receptor interaction into a biological response through different signalling pathways (adenylyl cyclase-cAMP or phospholipase C-phosphatidylinositol) are crucial properties of receptors. While many new receptors have been identified and thereafter characterized functionally during the 1980s, molecular biology now emerges as the privileged way for the structural characterization and discovery of receptors. Different strategies of receptor cloning have been developed which may or may not require prior receptor purification. Among cloning strategies that do not require receptor purification, homology screening of cDNA libraries, expression of receptor cDNA or mRNA in Xenopus laevis oocytes or in COS cells, and the polymerase chain reaction method achieved great success, e.g. cloning of receptors for cholecystokinin, gastrin, glucagon-like peptide 1, gastrin-releasing peptide/bombesin, neuromedin K, neuropeptide Y, neurotensin, opioids, secretin, somatostatin, substance K, substance P and vasoactive intestinal peptide. All these receptors belong to the superfamily of G-protein-coupled receptors which consist of a single polypeptide chain (350-450 amino acids) with seven transmembrane segments, an N-terminal extracellular domain and a C-terminal cytoplasmic domain. In this chapter, we have detailed the properties of three receptors which play an important role in digestive tract physiology and illustrate various signal transduction pathways: pancreatic beta-cell galanin receptors which mediate inhibition of insulin release and intestinal epithelial receptors for vasoactive intestinal peptide and peptide YY, which mediate the stimulation and inhibition of water and electrolyte secretion, respectively.

Characterization of the oligosaccharide moiety of VIP receptor from the human pancreatic cell line BxPC-3

The human pancreatic cell line BxPC-3 displays two classes of binding sites with high and low affinity for VIP. The order of potency of VIP-related peptides in inhibiting either [125I]VIP or [125I]N-AcPACAP27 binding and in stimulating cAMP production was typical of the human VIP receptor. By combining affinity labeling with glycosidase treatments, we have characterized the VIP receptor as a M(r) = 68,200 glycoprotein, consisting of a M(r) = 39,300 polypeptide core with at least three N-linked oligosaccharide chains. In addition, our results revealed the presence of a low amount of sialic acid residues in the carbohydrate moiety of receptor.

Glycosylation of VIP receptors: a molecular basis for receptor heterogeneity

Apparent molecular weights of VIP-binding proteins differ greatly according to species and to tissue. In this study, we used plasma membranes from various species (human, rat, pig) and tissues (melanoma, intestine, liver), which display major 125I-VIP-labeled components with molecular weights ranging from M(r) = 51,800 to 66,800. With the exception of porcine receptor, the various VIP receptors had similar apparent molecular weights after removal of their N-linked carbohydrates. In addition to differences in the amount of asparagine-linked glycans, our results also revealed differences in the composition of the oligosaccharide chains, which can also account for the heterogeneity in the molecular weights of the VIP receptor.

Effect of inhibiting N-glycosylation or oligosaccharide processing on vasoactive intestinal peptide receptor binding activity and structure

We used inhibitors of four steps of the glycosylation pathway to examine the contribution of carbohydrate moieties to the ligand-binding activity, cell-surface expression and apparent molecular mass of the human vasoactive intestinal peptide (VIP) receptor. Human melanoma IGR 39 cells, incubated for 60 h with the inhibitors tunicamycin, castanospermine, swainsonine or deoxymannojirimycin, under conditions where cell viability and protein synthesis were not affected, expressed VIP receptor species with different VIP-binding properties. The most pronounced effects on VIP binding were obtained with tunicamycin and deoxymannojirimycin, which respectively caused 80% and 67% inhibition. Treatment with either swainsonine or castanospermine resulted in only a 25-32% decrease in VIP specific binding. Based on Scatchard analyses of data from competition experiments, the decrease in VIP-binding activity in either swainsonine- or deoxymannojirimycin-treated cells was due to a decrease in ligand affinity; the cell-surface number of VIP-binding sites remained unchanged. In contrast, tunicamycin and castanospermine caused decreases in the cell-surface number of functional VIP receptors without affecting affinity. Besides, the drug-treated cells produced VIP-binding proteins with different molecular masses and endoglycosidase H (Endo H) sensitivities. When compared with their counterpart synthesized in control cells, VIP-binding proteins produced by deoxymannojirimycin- or swainsonine-treated cells were smaller in size and exhibited the expected sensitivity to Endo H. No modification in the apparent molecular mass was observed in the presence of either castanospermine or tunicamycin. In addition, after Endo F digestion, all of the deglycosylated proteins migrated with the same electrophoretic mobility. Finally, processing in the presence of castanospermine led to an Endo H-resistant receptor species which showed an unexpected neuraminidase-sensitivity, indicating that, as in control cells, these receptors carry V-linked oligosaccharides with terminal sialic acid residues.

Modulatory effects of VIP and related peptides from the gastrointestinal tract on cell mediated cytotoxicity against tumour cells in vitro

In the present study the effect of vasoactive intestinal peptide (VIP), peptide histidine-methionine (PHM), and secretin on spontaneous cell mediated cytotoxicity of peripheral blood mononuclear cells against tumour target cells was evaluated. VIP stimulated cytotoxicity against CaCo-2 human colon cancer cells, whereas less effect was seen against K-562 erythroleukemia cells. Depletion of CD16+ natural killer cells almost completely abolished cytotoxicity and subsequent VIP incubation did not change residual activity. In contrast to PHM, which hardly influenced cytotoxicity, secretin was found to be more effective especially against K-562 target cells. These observations suggest a modulating role for the neuropeptide VIP in the cellular immune response against tumour cells, especially from the colon, resulting in increased activity of CD16+ natural killer cells. Secretin, seems to be less potent in modulating cellular cytotoxicity. These findings support the concept that gastrointestinal peptides can play a role in the regulation of cellular cytotoxicity against tumor cells.

Characterization of VIP- and helodermin-preferring receptors on human small cell lung carcinoma cell lines

We investigated the molecular and pharmacologic characteristics of VIP receptors on two human SCLC cell lines: NCI-N592 and NCI-H345. With NCI-N592 cell, the order of potency of VIP-related peptides in inhibiting 125I-VIP binding and in stimulating cAMP production was typical of the human VIP receptor. By covalent cross-linking, a polypeptide of Mr 62,300 was obtained. Conversely, the behavior of NCI-H345 cell line was totally different: helodermin was the most potent peptide, VIP and PHI were equipotent, while hGRF and secretin were totally ineffective. These results suggest that NCI-N592 cells possess a typical VIP receptor while NCI-H345 cells possess a helodermin-preferring receptor, and that the natural target of helodermin might not be the VIP receptor.

A human melanoma-derived cell line (IGR39) with a very high number of vasoactive-intestinal-peptide (VIP) receptors. 1. Molecular characterization of the binding site

Using mono[125I]iodinated vasoactive intestinal peptide (125I-VIP), a very high number of specific binding sites for VIP were identified at the surface of the human melanoma cell line IGR39. The Scatchard analysis of competitive displacement experiments between native VIP and 125I-VIP was consistent with the existence of two classes of VIP-binding sites. IGR39 cells possess 0.54 x 10(6) high-affinity sites with a dissociation constant (Kd) of 0.66 nM and 1.3 x 10(6) sites of moderate affinity with a Kd of 4.7 nM. Pharmacological studies indicated that the order of potency in inhibiting 125I-VIP binding of the VIP/secretin family peptides was VIP much greater than peptide histidine methioninamide greater than human growth-hormone-releasing factor(1-44) greater than secretin. Glucagon has no effect on the binding of the labelled peptide. By means of photoaffinity labelling a polypeptide of Mr 63,000 was characterized. The labelling of this species was completely abolished by native VIP. The order of potency of VIP-related peptides in inhibiting 125I-VIP cross-linking to its receptor was the same as in the competition experiments. The glycoprotein nature of the VIP-binding sites of IGR39 cells has been investigated by affinity chromatography on wheat-germ-agglutinin-Sepharose.

The vasoactive intestinal peptide (VIP) receptor: recent data and hypothesis

Vasoactive intestinal peptide (VIP) is a neuropeptide with a broad range of biological activities in various tissues. After interaction with its membrane receptor, VIP generally induces a very large increase in the intracellular cyclic AMP level. Receptors for VIP have been described in numerous tissues and cell lines. The first results on VIP receptor structure have been obtained by covalent cross-linking using bifunctional reagents. The molecular mass of the different components characterized in this way differs greatly according to the species and the tissue used. This heterogeneity may reflect either a difference in the length of the cross-linked polypeptide backbone or differently glycosylated forms of the same polypeptide. The VIP binding site of intact human adenocarcinoma cells (HT29 cells) is an Mr 64,000 glycoprotein with 20kDa of N-linked oligosaccharide side chains containing sialic acid. The structure of the VIP binding site from HT29 cell is compared, first to the structure of the VIP receptor from other tissues, particularly that from rat liver, and second to the structure of the hepatic glucagon binding site. Recently, solubilization of the VIP receptor in an active form has provided a new way of studying this receptor. The HT29 cell line is an appropriate model to study the dynamics of the VIP receptor. After binding to its receptor, VIP is rapidly internalized, probably by receptor-mediated endocytosis. This internalization leads to a decrease in the cell surface receptor number and simultaneously to a homologous desensitization of adenylate cyclase. VIP is then degraded in the lysosomes, while most of the receptors are recycled back to the cell surface. The presence of an intracellular pool of unoccupied VIP receptors has been demonstrated after inactivation of the cell surface receptors by chymotrypsin. The kinetics of the receptor reappearance at the cell surface, after inactivation by chymotrypsin or after receptor-mediated endocytosis, indicate 2 possible intracellular pathways for occupied and unoccupied VIP receptors.

Molecular characteristics and evidence for internalization of vasoactive-intestinal-peptide (VIP) receptors in the tumoral rat-pancreatic acinar cell line AR 4-2 J

1. Vasoactive intestinal peptide (VIP) receptors were investigated in the tumoral acinar cell line AR 4-2 J derived from rat pancreas [125I]Iodo-VIP binding to cell membranes showed the following IC50 values for unlabeled peptides: VIP, 0.3 nM; peptide His-IleNH2, 2 nM; helodermin, 30 nM; secretin, 100 nM. After incubation with 20 nM dexamethasone, the binding capacity increased twofold but affinities were unchanged. External [125I]iodo-VIP binding to intact cells reached steady state after 5 min at 37 degrees C, while the sequestration-internalization of the [125I]iodo-VIP-receptor complex (tested by cold acid washing) increased progressively, reaching 75% of total binding after 1 h. This phenomenon was blocked at 4 degrees C. Further data with dexamethasone, tunicamycin, cycloheximide, low temperature, and/or phenylarsine oxide, suggested a half-life of 2 days for VIP receptors and the necessity of N-glycosylation for proper translocation. 2. For chemical [125I]iodo-VIP cross-linking bis[2-(succinimidooxycarbonyloxy)ethyl]sulfone gave the best yield when compared with five other bifunctional reagents. In membranes, the main specifically cross-linked peptide had Mr 66,000 under nonreducing conditions, and migrated with lower velocity (-5%) under reducing conditions. Cross-linking was suppressed by VIP, peptide His-IleNH2 and helodermin (competitively) and also by GTP. In intact cells, the Mr of [125I]iodo-VIP-cross-linked peptides depended on the mode of cell solubilization. After direct solubilization, the major cross-linked radioactivity migrated as a smear of Mr 130,000-180,000 but an Mr-66,000 peptide was also detectable. In contrast, the solubilization of cross-linked cells detached by mild trypsinisation gave mainly the Mr-66,000 labeled peptide. This suggests that most VIP receptors in intact, attached cells were in a high-Mr complex and that mild cell treatment was sufficient to disrupt this complex.

Glycoprotein nature of alpha 2-adrenergic receptors labeled with p-azido[3H] clonidine in calf retina membranes

alpha 2-Adrenergic receptors in calf retina membranes can be specifically labeled with the tritiated agonist p-azido[3H]clonidine. Saturation binding in the dark occurs with high affinity (1.3 +/- 0.3 nM) to a single class of sites (1122 +/- 67 fmol/mg protein). Irradiation of the membrane-bound radioligand results in the labeling of a peptide band with an apparent size of 65 kDa and a characteristic pharmacological profile for an alpha 2-adrenergic receptor. The carbohydrate moieties of the alpha 2-receptor are characterized by lectin affinity chromatography and glycosidase treatment. The Nonidet P-40-solubilized, p-azido[3H]clonidine-labeled receptors are completely retained by Con A- as well as WGA-Sepharose columns. Neuraminidase, alpha-mannosidase and TFMS do not affect the electrophoretic mobility of the receptor on SDS-PAGE whereas endoglycosidase F reduces the apparent size to 45 kDa.

Species differences in the molecular characteristics of vasoactive-intestinal-peptide receptors in the pancreas from rat and guinea-pig

1. The receptors for vasoactive intestinal peptide (VIP) present in dispersed acini and membranes from rat and guinea-pig pancreas differed in selectivity pattern, i.e. in the displacement of [125I]iodo-VIP by parent peptides, as revealed by a VIP:secretin IC50 ratio at least ten times higher in rat than in guinea-pig preparations. The molecular properties of these VIP receptors were therefore investigated. 2. When comparing six succinimidyl ester cross-linkers, bis[2-(succinimidooxycarbonyloxy)ethyl]sulfone proved to be the most universal [125I]iodo-VIP cross-linker for all pancreatic preparations. 3. In intact rat acini the main labeled peptide had an Mr of 80,000, whereas the main labeled peptide in intact guinea-pig acini was a smear of Mr 160,000. In both rat and guinea-pig pancreatic membranes, the main labeled peptide ([125I]iodo-VIP-binding-protein complex) had an Mr of 66,000. In addition, variable proportions of an Mr-80,000 peptide and an Mr-83,000 peptide were visualized in, respectively, rat and guinea-pig membranes. The labeling of all peptides was suppressed by VIP and by GTP. Reducing conditions allowed only a better resolution, making the presence of intermolecular disulfide bridges unlikely. 4. Taking into account the Mr of VIP it is thus plausible that the main native Mr-77,000 VIP-binding site present in rat acini could be easily converted to an Mr-63,000 peptide during membrane preparation, while in guinea-pig acini Mr-80,000 and/or Mr-63,000 VIP-binding sites were often closely associated with another membrane component in the native state. These molecular differences between VIP receptors in intact rat and guinea-pig acini are in keeping with functional differences.

Photoaffinity labelling of the vasoactive-intestinal-peptide-binding site on intact human colonic adenocarcinoma cell line HT29-D4. Synthesis and use of photosensitive vasoactive-intestinal-peptide derivatives

N-Hydroxysuccinimidyl 4-azidobenzoate, a u.v.-sensitive heterobifunctional reagent, was used to synthesize photoreactive derivatives of the vasoactive intestinal peptide (VIP). Products of the reaction were purified by reverse-phase h.p.l.c. Three 4-azidobenzoyl-VIP (4-AB-VIP) derivatives were able to compete with monoiodinated 125I-VIP with an apparent KD of 2.5, 6.3 and 12.5 nM compared with 0.6 nM for native VIP. H.p.l.c.-purified mono[125I]iodinated VIP was used to synthesize 4-AB-125I-VIP derivatives. They were used to photoaffinity-label the VIP-binding site of HT29-D4 cells, a clone derived from the human colonic adenocarcinoma cell line HT29. Only one polypeptide, of Mr 70,000 +/- 5000 (mean +/- S.D.) was specifically labelled. The Mr of the component thus characterized was slightly higher than that of the major species (Mr 67,000) labelled after cross-linking experiments using 125I-VIP, conventional homobifunctional reagents and HT29 cells. Nevertheless, the specificity and extent of glycosylation of these two components were identical. These new photosensitive VIP derivatives should be useful tools with which to investigate further VIP-receptor structure and metabolism.

Neuropeptide modulation of the immune response in gut associated lymphoid tissue

The neuropeptides vasoactive intestinal peptide (VIP), substance P, and somatostatin are found in high concentrations in both the central nervous system and the gastrointestinal tract. Specific high affinity receptors for VIP, substance P and somatostatin have been identified on both human and murine lymphocytes, suggesting a role for each of these neuropeptides in a neuroimmune axis. These peptides may be important modulators of mucosal immunity regulating lymphocyte proliferation and trafficking in gut associated lymphoid tissue, synthesis of IgA, and histamine release. Somatostatin antagonism of both VIP and substance P effects has been observed in the immune system. Though the mechanisms by which these neuropeptides modulate immune function have not been completely delineated, current evidence supports the hypothesis that VIP modulates immune function via cAMP dependent pathways while substance P regulation of the immune response involves phospholipid metabolism. Somatostatin inhibition of both cAMP dependent and phospholipid dependent effects has been documented in endocrine tissues. Delineation of the role of these peptide-peptide interactions in modulation of the immune response promises to be a fruitful area for further investigation.

Identification of nuclear receptors for VIP on a human colonic adenocarcinoma cell line

Vasoactive intestinal peptide (VIP) is a neuropeptide with broad tissue distribution. Although its precise function is unknown, it is thought to exert its effect, at least in part, by interacting with cell surface receptors. Nuclear receptors for VIP have now been identified by specific binding of 125I-labeled VIP to nuclei of a human colonic adenocarcinoma cell line (HT29) and by cross-linking of 125I-labeled VIP to its receptor on intact nuclei. In contrast, 125I-labeled transferrin shows only background binding to nuclei but significant binding to intact cells. Purity of the isolated nuclei was further substantiated by electron microscopy. The apparent molecular sizes of the VIP--cross-linked nuclear and cell surface receptors are similar but not identical.