Vasoactive intestinal peptide receptor on liver plasma membranes: characterization as a glycoprotein

Vasoactive intestinal peptide induces proliferation of human hepatocytes

OBJECTIVES

Proliferation of hepatocytes in vitro can be stimulated by growth factors such as epidermal growth factor (EGF), but the role of vasoactive intestinal peptide (VIP) remains unclear. We have investigated the effect of VIP on maintenance and proliferation of human hepatocytes.

MATERIALS AND METHODS

Human hepatocytes were isolated from liver specimens obtained from patients undergoing liver surgery. Treatment with VIP or EGF was started 24 h after plating and continued for 3 or 5 d. DNA replication was investigated by Bromodeoxyuridine (BrdU) incorporation and cell viability detected by MTT assay. Cell lysate was analysed by western blotting and RT-PCR. Urea and albumin secretion into the culture supernatants were measured.

RESULTS

VIP increased DNA replication in hepatocytes in a dose-dependant manner, with a peak response at day 3 of treatment. VIP treatment was associated with an increase in mRNA expression of antigen identified by monoclonal antibody Ki-67 (MKI-67) and Histone Cluster 3 (H3) genes. Western blotting analysis showed that VIP can induce a PKA/B-Raf dependant phosphorylation of extracellular signal-regulated kinases (ERK). Although EGF can maintain hepatocyte functions up to day 5, no marked efffect was found with VIP.

CONCLUSIONS

VIP induces proliferation of human hepatocytes with little or no effect on hepatocyte differentiation. Further investigation of the role of VIP is required to determine if it may ultimately support therapeutic approaches of liver disease.

Comitogenic effects of vasoactive intestinal polypeptide on rat hepatocytes

The primary mitogens such as epidermal growth factor and transforming growth factor-alpha are known to stimulate DNA synthesis in primary cultures of adult rat hepatocytes. Vasoactive intestinal polypeptide (VIP) was found to amplify DNA synthesis induced by the primary mitogens and thus acted as a comitogen. The comitogenic effect of VIP was specific for the culture medium, suggesting that minor components in the medium were required for hepatocytes to fully respond to VIP. Glutamic acid is probably one of these minor components, although other components present in the nutrient-rich medium were also necessary for the full comitogenic effect. Other comitogens such as insulin, vasopressin, and angiotensin II interacted additively with low concentrations of VIP. The comitogenic effect of VIP was also found in hepatocytes cultured from regenerating rat liver after a partial hepatectomy. In the regenerating hepatocyte cultures, VIP can act as a mitogen even in the absence of the primary mitogen EGF. VIP mRNA was found in several organs including brain, intestine, and liver, and its expression was slightly induced in liver 24 h after a partial hepatectomy. These results suggest that VIP can act as a hepatic comitogen and may play a role in liver cell proliferation.

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.

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.

Vasoactive intestinal peptide receptors in rat liver after partial hepatectomy

We describe the status of vasoactive intestinal peptide (VIP) receptors in regenerating liver. VIP-stimulated adenylate cyclase activity was markedly decreased in proliferating liver 3 days after partial (70%) hepatectomy. This was associated with a reduced efficacy of VIP (53% compared with controls), with no change in the potency of the peptide (ED50 0.8 nM). In contrast, forskolin- and guanosine 5'-[beta gamma-imido]triphosphate (Gpp[NH]p)-stimulated enzyme activities were not decreased after hepatectomy. The expression of Gs protein subunits (alpha and beta) was studied by cholera toxin-catalysed ADP ribosylation of alpha s and by immunoblotting of alpha s and beta subunits. Both subunits were increased in regenerating liver, further suggesting that the decreased response to VIP was not related to a decreased expression of Gs proteins. In fact, the reduced adenylate cyclase response to VIP in regenerating liver was associated with quantitative and structural changes in VIP receptors. Equilibrium binding data obtained with 125I-VIP indicated the presence of two classes of binding sites, the Kds of which were not altered after hepatectomy. In contrast, changes in binding capacity (Bmax.) were as follows: 0.11 +/- 0.01 and 0.05 +/- 0.01 pmol/mg of protein for high-affinity sites in control and hepatectomized rats respectively; and 2.3 +/- 0.2 and 0.65 +/- 0.03 pmol/mg of protein for low-affinity sites in control and hepatectomized rats respectively. Moreover, affinity labelling experiments showed that the M(r) value of 125I-VIP-receptor complexes was higher in regenerating liver than in quiescent hepatocytes, e.g. 58,000 and 53,000 respectively. It is concluded that VIP receptors are altered in regenerating liver, resulting in a decreased response of adenylate cyclase to the neuropeptide.

Regional distribution of guanine nucleotide-sensitive and guanine nucleotide-insensitive vasoactive intestinal peptide receptors in rat brain

Based on the ability of guanine nucleotides to inhibit the binding of vasoactive intestinal peptide to its receptors, a guanosine 5'-triphosphate analog, guanylyl-imidodiphosphate, was used to differentiate two subtypes (or different functional states of a single subtype) of vasoactive intestinal peptide receptor in brain with in vitro autoradiography. In most brain regions, guanylyl-imidodiphosphate reduced vasoactive intestinal peptide binding between 40 and 60%. However, in the supraoptic nucleus, locus coeruleus, interpeduncular nucleus, facial nucleus, olfactory tubercle and periventricular hypothalamic nucleus, 80% or more of vasoactive intestinal peptide binding was inhibited. In other brain regions, including the medial geniculate, olfactory bulbs, and ventral thalamic nuclei, guanylyl-imidodiphosphate had little effect on vasoactive intestinal peptide binding. In liver, lung and intestine it also partly inhibited vasoactive intestinal peptide binding. Electrophoretic analysis of vasoactive intestinal peptide, covalently cross-linked to its receptors in brain membranes, revealed a pair of bands between 44,000 and 52,000 mol. wt, a component at 64,000 mol. wt and another at 92,000 mol. wt. All were displaceable with vasoactive intestinal peptide but guanylyl-imidodiphosphate displaced only the 64,000 mol. wt band suggesting that the GTP-sensitive vasoactive intestinal peptide receptor seen in brain sections has a molecular weight of about 61,000. The differential sensitivity to guanylyl-imidodiphosphate suggests the existence of at least two vasoactive intestinal peptide receptor subtypes in brain, with distinct regional distribution, and may reflect differential coupling to second messenger systems.

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.

Vasoactive intestinal peptide receptors on AR42J rat pancreatic acinar cells

VIP receptors on AR42J rat pancreatic cells were analyzed by competition binding, affinity labeling and by N-glycanase digestion analyses. These studies revealed the presence of specific, high affinity (Kd approximately 1 nM) VIP receptors with a mass of 67 kDa or 59 kDa under reducing or non-reducing conditions, respectively. N-glycanase digestion of affinity labeled membranes generated a core receptor protein of approximately 44 kDa and evidence for at least two N-linked glycans on the mature receptor. The receptor lacked O-linked oligosaccharides but contained terminal sialic acid residues on its N-linked glycan(s) based on digestions with O-glycanase and neuraminidase. The similarity of the AR42J VIP receptor to the recently cloned cDNA for human VIP receptors makes this cell line an attractive model for further analysis of VIP receptor signal transduction events.

Epithelial cells purified from choroid plexus have receptors for vasoactive intestinal polypeptide

Using the choroid plexus from pig a method has been developed to purify the epithelial cells from the underlying vascularized connective tissue stroma. An epithelial cell fraction was obtained that showed a purity of at least 95%, as determined by light microscopic analysis. The epithelial cells were investigated for the presence of binding sites for the neurotransmitter peptide, vasoactive intestinal polypeptide (VIP). Suspensions of epithelial cells were found to have high affinity binding sites for 125I-labelled VIP, with maximum binding obtained after 30 min incubation at 20 degrees C with a concentration of 50 micrograms cell protein per sample. Competition experiments with displacement of [125I]VIP binding by increasing concentrations of unlabeled VIP indicated the presence of a single class of binding sites with a Kd of 3 nM and a binding capacity of 970 pmol/g cell protein. Cross-linking of [125I]VIP to epithelial cells with disuccinimido dithiobis (propionate) (DSP), followed by SDS-polyacrylamide gel electrophoresis, demonstrated binding to a single 55 kD protein. The receptor was highly specific for VIP as binding was only inhibited in the presence of high concentrations of the related peptides helodermin, growth hormone-releasing factor, secretin, and peptide histidine isoleucine. This is the first demonstration of VIP-binding to choroid plexus epithelial cells.

Characterization of vasoactive intestinal peptide receptors in rat seminal vesicle

Receptors for vasoactive intestinal peptide (VIP) in membranes from rat seminal vesicle were examined using 125I-labeled VIP as ligand. The receptor binding was rapid, reversible, saturable, specific, and dependent on temperature and membrane concentration. At 15 degrees C, the stoichiometric data suggested the presence of two classes of VIP receptors with Kd values of 0.54 and 44.4 nM and binding capacities of 73 and 1,065 fmol VIP/mg membrane protein, respectively. The interaction showed a high degree of specificity, as suggested by competition experiments with various peptides structurally related to VIP as follows: helodermin was 10 times, secretin 30 times, and rat growth hormone-releasing factor 300 times less potent than VIP, whereas glucagon did not recognize VIP receptors in concentrations of up to 10 microM. The binding of 125I-VIP to membranes was sensitive to the presence of GTP in the incubation medium in a dose-dependent manner. To characterize the molecular weight of these VIP receptors, 125I-VIP was covalently bound to membranes from rat seminal vesicle using dithiobis(succinimidyl propionate); sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the solubilized receptor revealed the presence of a specific component with a molecular mass of 47,000 Da as estimated in denaturing conditions. These findings, together with the known presence of VIP-containing nerves in the seminal vesicle, suggest a direct physiological role for this peptide in this accessory gland of the male genital tract.

Molecular characterization of helodermin-preferring VIP receptors in SUP T1 lymphoma cells: evidence for receptor glycosylation

Cross-linking of [125I]helodermin to human SUP-T1 lymphoblasts with bis[2-(succinimidooxycarbonyloxy)ethyl]sulfone (BSOCOES) revealed a 63 K binding protein. This cross-linking was inhibited by helodermin and VIP. In cells submitted for 3-4 days to 0.2 microgram/ml tunicamycin, the Mr of an increasing proportion of helodermin-preferring receptors was reduced to 50 K and the total number of receptors was decreased by about 50%, without alteration in binding affinity and specificity. In parallel, the VIP-mediated adenylate cyclase stimulation was reduced by 30% with no change in NaF-, Gpp[NH]p-, and PGE1-stimulations. We conclude that a proper N-glycosylation of helodermin-preferring VIP receptors is required for normal receptor targeting and turnover but not for ligand binding and adenylate cyclase coupling.

Vasoactive intestinal peptide receptor on liver plasma membranes: solubilization and cross-linking

The hepatic receptor for VIP was solubilized from rat liver plasma membranes with 1.4% digitonin and shown to conserve its ability to bind to the ligand. This solubilized receptor demonstrated the high affinity and specificity for VIP (KD approximately 1 nM, binding preference: VIP greater than PHI greater than secretin greater than thymosin alpha 1) which were observed with the nonsolubilized VIP receptor on intact liver plasma membranes. 125I-VIP was next cross-linked to either the solubilized or nonsolubilized receptor using disuccinimido suberate or disuccinimido dithiobis(propionate), and the resulting complexes analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by autoradiography. A broad autoradiographic band which demonstrated a high affinity for VIP was identified at Mr 56,000 (53,000 in the absence of the reducing agent dithiothreitol) for both the solubilized and nonsolubilized receptors. We have thus been able to solubilize from rat liver plasma membranes a receptor with high affinity and specificity for VIP, and confirmed its structural similarity with the native VIP receptor in nonsolubilized membranes using cross-linking techniques.

Comparison of the neuropeptide Y receptor in the rat brain and intestine

Neuropeptide Y (NPY) is widely distributed in the central and peripheral nervous systems where it serves neuromodulator and neurotransmitter functions. NPY is contained within intrinsic nerves of the small intestine and can be demonstrated to inhibit intestinal secretion when added to the serosal side of intestine mucosa mounted in Ussing chambers. When injected centrally it has potent effects on food intake, blood pressure, sexual activity and circadian rhythms. Using NPY radiolabeled with iodogen, lactoperoxidase, or the Bolton-Hunter reagent, we have localized high-affinity NPY receptors on brain membranes and on the serosal laterobasal membranes of the rat intestinal epithelial cell. We have demonstrated that enzymatic degradation may limit the ability to demonstrate NPY binding to brush border membranes. In other experiments NPY was cross-linked to its receptors in brain and intestine using disuccinimido suberate and the resulting complexes analyzed on SDS polyacrylamide gel electrophoresis followed by radioautography. We identified two main NPY receptor species in the intestine with molecular sizes of 52-59 kDa and 37-39 kDa. The 37-39 kDa species may possess a disulfide bond which gives the receptor a fixed conformation, or it may be composed of two subunits (37-39 kDa and approximately 5 kDa subunits). This conclusion is based on the different migration of the smaller band in the presence of the reducing agent, dithiothreitol. The intestinal NPY receptor exhibits differences from the rat brain receptor previously characterized by us using similar techniques. The brain receptor has a molecular weight of approximately 58 kDa with a smaller species of about 35 kDa which shows no differences in migration after exposure to dithiothreitol. The localization of NPY receptors on laterobasal membranes and brain membranes is consistent with previous anatomic and physiologic findings. The different characteristics of each receptor type provides physical evidence of receptor heterogeneity. However, it is possible that the greater enzymatic degradation observed in intestinal membranes might explain the differences in receptor sizes in the two organs.

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.

Characterization of vasoactive intestinal peptide receptors in retina

Vasoactive intestinal peptide (VIP) is a neuropeptide having a wide range of effects on a large number of tissues. To gain insight into the role VIP plays in retinal function, VIP receptors in bovine retinal membranes were analyzed in competition binding assays and by affinity labeling studies and compared to VIP receptors in rat liver membranes. In both membrane preparations, high affinity VIP binding sites (KD approximately 1 nM) were detected. Secretin and glucagon, each having close structural homology to VIP, were found to have negligible effects on [125I]VIP binding in retina. In contrast, secretin (KD = 70 nM) was modestly effective in inhibiting [125I]VIP binding to rat liver membranes. Affinity labeling analysis revealed a VIP binding site of 59 kDa in both bovine retinal and rat liver membranes. Digestion of affinity-labeled receptor proteins with endoglycosidase F generated final cleavage products of approx. 45 kDa for both receptors. These results indicate that the retina expresses a high affinity, highly selective VIP receptor thereby supporting a specific function for VIP in this tissue.

Characterization of the detergent solubilized receptor for gastrin-releasing peptide

Properties of detergent solubilized gastrin-releasing peptide receptor were investigated. Swiss 3T3 membranes were covalently labeled with [125I]GRP and homobifunctional cross-linkers. A major labeled protein of 75 kDa was resolved using SDS-polyacrylamide gel electrophoresis. When the same preparation was solubilized with zwitterionic detergent and analyzed under nondenaturing conditions the protein bound radioactivity was resolved in two different peaks, a major one of apparent molecular weight 220,000 (peak 1) and a minor one of 80,000 (peak 2) both containing the 75 kDa protein. Specific ligand binding activity also eluted with peak 1. These results indicate that the active form of bombesin/GRP receptor is a large complex containing the 75 kDa ligand binding domain.

Functional and immunological evidence for stable association of solubilized vasoactive-intestinal-peptide receptor and stimulatory guanine-nucleotide-binding protein from rat liver

We have reported the solubilization of complexes between vasoactive intestinal peptide (VIP) and its receptor from rat liver in a GTP-sensitive form of Mr 150,000 [Couvineau, A., Amiranoff, B. & Laburthe, M. (1986) J. Biol. Chem. 261, 14482-14489]. In the present study, we demonstrate a stable association of solubilized VIP receptor and stimulatory guanine nucleotide-binding protein (Gs protein), taking advantage of the ability of the glycoproteic VIP receptor (Mr 48,000), and the inability of the Gs protein, to adsorb to wheat germ agglutinin (WGA). 125I-VIP-receptor complexes solubilized in Triton X-100 were adsorbed on WGA-Sepharose, extensively washed and the radioactivity retained was eluted with 1 mM GTP showing that: (a) radioactivity corresponds to free 125I-VIP and (b) alpha s (Mr 42,000) and beta (Mr 35,000) subunits of Gs protein are detectable in the GTP eluate by immunoblotting using antisera against these subunits. Such an effect of GTP implied that a stable ternary complex consisting of VIP, receptor and Gs protein had been adsorbed to WGA-Sepharose. When Triton-solubilized 125I-VIP-receptor complexes were adsorbed on WGA-Sepharose, then retained material was specifically eluted with 0.3 M N-acetylglucosamine, analysis of the sugar eluate showed the following results. (a) GTP induces the dissociation of 125I-VIP-receptor complexes of Mr 150,000 contained in the eluate indicating that 125I-VIP-receptor-G protein complexes had been adsorbed to the WGA column. (b) The Mr-42,000 alpha s subunit can be specifically ADP-ribosylated by cholera toxin. (c) Immunoblotting using antisera against the alpha s and beta subunits of Gs protein, reveals Mr-42,000 and Mr-35,000 components corresponding to alpha s and beta subunits, respectively. (d) Affinity cross-linking using dithiobis(succinimidyl-propionate) of 125-I-VIP-receptor complexes eluted from the WGA column reveals a major band corresponding to Mr 150,000. Immunoblotting using antisera against the beta-subunit shows the presence of the beta subunit (Mr 35,000) in this Mr-150,000 component. In conclusion, these data provide functional and immunochemical evidence for the physical association of solubilized VIP-receptor complexes with alpha s and beta subunits of Gs protein.

Cholecystokinin binding and degradation by isolated rat liver cells

The present studies were directed to examine and quantify binding and degradation of radiolabelled cholecystokinin (CCK) peptides by isolated rat liver cells. After incubation with liver cells (4.5 x 10(6) cells/ml) at 14 degrees C, minimal binding (less than 5%) of labelled CCK33 was detected. When labelled nonsulfated (nsCCK8) and sulfated CCK8 (sCCK8) were incubated, 16.2 +/- 1.8% (mean +/- S.E.) and 7.2 +/- 0.1% of 125I-nsCCK8 and 125I-sCCK8, respectively, were bound to the cell fraction. However, no inhibition of binding of either labelled nsCCK8 or sCCK8 was observed when incubated in the presence of excess unlabelled peptide (10 ng-10 micrograms). Preferential binding of labelled sCCK8, the biologically active form of the octapeptide, appeared to be to the nonparenchymal liver cell, rather than the hepatocyte, fraction; when corrected for cell size and protein content, binding of sCCK8 was approximately 15-times greater by the nonparenchymal cell population. When incubated with hepatocytes at 37 degrees C for 60 min, no degradation of labelled sCCK8 was detected by high pressure liquid chromatography. In contrast, progressive degradation of sCCK8 was observed when the peptide was incubated with the nonparenchymal cells. The results of these studies confirm previous observations that CCK33 is not bound by the liver. They further demonstrate that to some degree CCK8 is preferentially bound and degraded by hepatic nonparenchymal cells; however, this binding appears to be noncompetitive and, therefore, probably not receptor-mediated.

Solubilization of active and stable receptors for vasoactive intestinal peptide from rat liver

Vasoactive intestinal peptide (VIP) receptors were solubilized from rat liver using the zwitterionic detergent CHAPS. Optimal conditions of solubilization were obtained with 5 mM CHAPS and 2.5 mg protein/ml. The binding of 125I-VIP to CHAPS extracts was time- and pH-dependent, saturable and reversible. The following order of potency of unlabeled VIP-related peptides for inhibiting 125I-VIP binding was observed: VIP greater than helodermin greater than peptide histidine isoleucine amide (PHI) greater than rat growth hormone releasing factor (rGRF) greater than secretin. This peptide specificity is identical to that of rat liver membrane-bound receptors. VIP binding activity in the CHAPS extract was destroyed by trypsin or dithiothreitol in accordance with the known sensitivity of membrane-bound receptors to these agents. VIP receptors in CHAPS extracts were stable for at least 5 days at 4 degrees C. Scatchard analysis of equilibrium binding data indicated the presence in CHAPS extracts of high (H) and low (L) affinity binding sites with the following characteristics: KdH = 0.27 nM and BmH = 34 fmol/mg protein; KdL = 51 nM and BmL = 1078 fmol/mg protein. The guanine nucleotide GTP inhibited 125I-VIP binding to soluble receptors and enhanced the dissociation of soluble VIP-receptor complexes, suggesting that GTP-binding proteins were functionally associated with VIP receptors in solution. Gel filtration of solubilized VIP receptors on Sephacryl S-300 revealed a single binding component with a Stokes radius of 6.1 nm. It is concluded that active VIP receptors can be extracted from liver membranes by CHAPS. The availability of this CHAPS-soluble, stable and functional receptor from a tissue which can be obtained in large amounts represents a major step toward the purification of VIP receptors.

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.

Membrane receptors in the gastrointestinal tract

This review focusses on the roles that membrane receptors and their transducers play in the physiology and pathology of the gastrointestinal tract. The multifactorial regulation of [correction] mucosal growth and function is discussed in relation to the heterogeneity of exocrine and endocrine populations that originate from progenitor cells in stomach and intestine.

Vasoactive intestinal peptide receptors in pancreas and liver. Structure-function relationship

In purified rat pancreatic plasma membranes, (D-Phe4)PHI interacts as a selective VIP agonist for rat pancreatic VIP-preferring receptors, based on binding selectivity and adenylate cyclase activation, therefore allowing us to discriminate between the participation of VIP-preferring and secretin-preferring receptors in VIP stimulation. VIP-preferring receptors also bind GRF. They rely on disulfide bridges for their functional integrity. Their coupling with adenylate cyclase, based on the intrinsic activity of VIP analogues, is poor when compared to that of hepatic VIP receptors. In fresh rat liver plasma membranes, high-affinity VIP receptors are specifically labeled with [125I]helodermin and [125I]His1, D-Ala NLeu27)GRF and are well coupled to adenylate cyclase while low-affinity VIP receptors are not. The first subtype of VIP receptors is highly responsive to guanyl nucleotides and is easily altered by dithiothreitol. Only after freezing and thawing are low-affinity hepatic VIP receptors coupled to adenylate cyclase. Concerning the chemical characterization of VIP receptors, 66- and 35-kDa peptides are detected after specific [125I]VIP cross-linking with double agents in rat pancreatic membranes. In contrast, in intact pancreatic acini, the main source of radioactivity has a molecular mass of 130-180 kDa (with no contribution of intramolecular disulfide bridges), and an 80-kDa peptide is also detectable. The 66-kDa species in membranes can conceivably derive from the 80-kDa species observed in intact cells. Its molecular mass is higher than that of the 56-kDa [125I]VIP cross-linked protein previously observed in rat liver membranes. Besides, species differences between rat and guinea pig pancreas are also evident.

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.

Peptide T from human immunodeficiency virus envelope does not interact with hepatic, intestinal and colonic vasoactive intestinal peptide (VIP) receptors

Acquired immunodeficiency syndrome (AIDS) is initiated by the attachment of the human immunodeficiency virus (HIV) to specific target cells. An octapeptide sequence contained within the envelope of HIV, peptide T, mediates the viral binding. Since there is a considerable structural homology between peptide T and VIP, it has been proposed that the VIP receptor may be the naturally occurring protein which provides the corresponding cellular attachment site. In three different models (rat intestinal epithelial cell membranes, rat liver plasma membranes and human colonic cells), we document the lack of interaction between peptide T and the VIP receptor. These observations would also exclude any pathophysiologic effect caused by the crossreactivity of peptide T or its analogues and these VIP receptors.

Autoradiographic distribution of vasoactive intestinal polypeptide receptors in rabbit and rat small intestine

Vasoactive intestinal peptide (VIP) is found in the enteric nervous system of all layers of the small intestine. In the gastrointestinal tract, VIP receptors coupled to adenylate cyclase are present on epithelial, smooth muscle and possibly mononuclear cells. This study analyzes the distribution of VIP binding using in vitro autoradiographic techniques. VIP binding was present in high density in the mucosal layer of rabbit duodenum, jejunum and ileum. Low VIP binding was noted over the smooth muscle layers or the lymphoid follicles. Similar results were obtained in rat small intestine. The density of VIP binding was greatest in duodenal mucosa but was present in lower density in jejunal and ileal mucosa. Again, low VIP binding was noted in the smooth muscle layers or lymphoid follicles. Thus, autoradiographic maps of small intestine indicate that VIP receptors are found primarily in the small intestinal mucosa.

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.

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

We have previously shown that the mono [125I]iodinated vasoactive intestinal peptide (125I-VIP) could be covalently cross-linked on intact colonic adenocarcinoma cells (HT29). A major Mr 67,000 and a minor Mr 120,000 cross-linked polypeptides have been characterized [Muller, Luis, Fantini, Abadie, Giannellini, Marvaldi & Pichon (1985) Eur. J. Biochem. 151, 411-417]. The glycoprotein nature of these species was investigated using endo-beta-acetylglucosaminidase F (Endo F) treatment, enzymic and chemical desialylation and wheat germ agglutinin (WGA)-Sepharose affinity chromatography. Affinity-labelled VIP-binding proteins solubilized by Nonidet P-40 bound to WGA-Sepharose and could be eluted specifically with N-acetyl-D-glucosamine. Treatment with Endo F resulted in an increased electrophoretic mobility of both polypeptides. The major and the minor VIP-binding proteins were converted respectively into Mr 47,000 and 100,000 species, indicating removal of 20 kDa of N-linked oligosaccharides. Deglycosylation with trifluoromethanesulphonic acid also led to a 20 kDa loss in mass of the Mr 67,000 component, indicating the absence of additional O-linked sugars on this polypeptide. The presence of sialic acid on the major VIP-binding protein was demonstrated after treatment of intact cells with neuraminidase or by chemical desialylation with hydrochloric acid. We conclude from this study that the VIP receptor from intact HT29-D4 cells is a glycoprotein with N-linked oligosaccharide side chains containing sialic acid.