Inhibitor of apoptosis protein expression in glioblastomas and their in vitro and in vivo targeting by SMAC mimetic GDC-0152.

Glioblastomas (GBMs) are the most aggressive primary brain tumors in adult and remain a therapeutic challenge. Targeting key apoptosis regulators with the ultimate aim to restore apoptosis in tumor cells could be an interesting therapeutic strategy. The inhibitors of apoptosis proteins (IAPs) are regulators of cell death and represent attractive targets, especially because they can be antagonized by SMAC mimetics. In this study, we first investigated the expression of cIAP1, cIAP2, XIAP and ML-IAP in human GBM samples and in four different cell lines. We showed that all GBM samples and GBM cell lines expressed all these IAPs, although the expression of each IAP varied from one case to another. We then showed that high level of ML-IAP predicted worse progression-free survival and overall survival in both univariate and multivariate analyses in two independent cohorts of 58 and 43 primary human GBMs. We then used GDC-0152, a SMAC mimetic that antagonizes these IAPs and confirmed that GDC-0152 treatment in vitro decreased IAPs in all the cell lines studied. It affected cell line viability and triggered apoptosis, although the effect was higher in U87MG and GL261 than in GBM6 and GBM9 cell lines. In vivo, GDC-0152 effect on U87MG orthotopic xenografts was dose dependent; it postponed tumor formation and slowed down tumor growth, significantly improving survival of GBM-bearing mice. This study revealed for the first time that ML-IAP protein expression correlates with GBM patient survival and that its antagonist GDC-0152 improves outcome in xenografted mouse.

[Selectins as adhesion molecules and potential therapeutic target]. / Les selectines: acteurs de l'adherence cellulaire et potentiel cible therapeutique.

Selectins belong to the family of adhesion molecules that recognize sugars as ligands through their Carbohydrate Recognition Domain (CRD). There are three types of selectin: the L-selectin (CD62L), which is constitutively expressed by most leukocyte populations, the P-selectin (CD62P) is found on activated platelets and endothelial cells, and the E-selectin (CD62E) expressed by activated endothelial cells. These three molecules exhibit high homology in their structures. Selectin-ligand interactions are among the most studied protein-glycan interactions in biology. The selectins and theirs ligands are involved in regulating inflammatory and immunological events that occur at the interface of the bloodstream and vessel walls. Their molecular partners are surface glycoconjugates harboring groups of the sialyl-Lewis antigens. This review presents an inventory of our current knowledge on the structures and functions of selectins and their ligands. We also provide an update on their involvement in pathophysiological processes, especially during inflammation and tumor development.

Unique disulfide bond structures found in ST8Sia IV polysialyltransferase are required for its activity.

NCAM polysialylation plays a critical role in neuronal development and regeneration. Polysialylation of the neural cell adhesion molecule (NCAM) is catalyzed by two polysialyltransferases, ST8Sia II (STX) and ST8Sia IV (PST), which contain sialylmotifs L and S conserved in all members of the sialyltransferases. The members of the ST8Sia gene family, including ST8Sia II and ST8Sia IV are unique in having three cysteines in sialylmotif L, one cysteine in sialylmotif S, and one cysteine at the COOH terminus. However, structural information, including how disulfide bonds are formed, has not been determined for any of the sialyltransferases. To obtain insight into the structure/function of ST8Sia IV, we expressed human ST8Sia IV in insect cells, Trichoplusia ni, and found that the enzyme produced in the insect cells catalyzes NCAM polysialylation, although it cannot polysialylate itself ("autopolysialylation"). We also found that ST8Sia IV does not form a dimer through disulfide bonds. By using the same enzyme preparation and performing mass spectrometric analysis, we found that the first cysteine in sialylmotif L and the cysteine in sialylmotif S form a disulfide bridge, whereas the second cysteine in sialylmotif L and the cysteine at the COOH terminus form a second disulfide bridge. Site-directed mutagenesis demonstrated that mutation at cysteine residues involved in the disulfide bridges completely inactivated the enzyme. Moreover, changes in the position of the COOH-terminal cysteine abolished its activity. By contrast, the addition of green fluorescence protein at the COOH terminus of ST8Sia IV did not render the enzyme inactive. These results combined indicate that the sterical structure formed by intramolecular disulfide bonds, which bring the sialylmotifs and the COOH terminus within close proximity, is critical for the catalytic activity of ST8Sia IV.

Exploring the acceptor substrate recognition of the human beta-galactoside alpha 2,6-sialyltransferase.

Human beta1,4-galactoside alpha2,6-sialyltransferase I (ST6GalI) recognition of glycoprotein acceptors has been investigated using various soluble forms of the enzyme deleted to a variable extent in the N-terminal half of the polypeptide. Full-length and truncated forms of the enzyme have been investigated with respect to their specificity for a variety of desialylated glycoproteins of known complex glycans as well as related proteins with different carbohydrate chains. Differences in transfer efficiency have been observed between membrane and soluble enzymatic forms, indicating that deletion of the transmembrane fragment induces loss of acceptor preference. No difference in substrate recognition could be observed when soluble enzymes of similar peptide sequence were produced in yeast or mammalian cells, confirming that removal of the membrane anchor and heterologous expression do not alter enzyme folding and activity. When tested on free oligosaccharides, soluble ST6GalI displayed full ability to sialylate free N-glycans as well as various N-acetyllactosaminyl substrates. Progressive truncation of the N terminus demonstrated that the catalytic domain can proceed with sialic acid transfer with increased efficiency until 80 amino acids are deleted. Fusion of the ST6GalI catalytic domain to the N-terminal half of an unrelated transferase (core 2 beta1,6-N-acetylglucosaminyltransferase) further showed that a chimeric form of broad acceptor specificity and high activity could also be engineered in vivo. These findings therefore delineate a peptide region of approximately 50 amino acids within the ST6GalI stem region that governs both the preference for glycoprotein acceptors and catalytic activity, thereby suggesting that it may exert a steric control on the catalytic domain.

alpha(1,2)-fucosylation prevents sialyl Lewis x expression and E-selectin-mediated adhesion of fucosyltransferase VII-transfected cells.

E-selectin is a cytokine-inducible, calcium-dependent endothelial cell adhesion molecule that plays a critical role in the leucocyte-endothelium interaction during inflammation and is thought to contribute to the metastatic dissemination of tumour cells. Like the other selectins, E-selectin binds to ligands carrying the tetrasaccharide sialyl-Lewis x (NeuAcalpha2,3Galbeta1,4[Fucalpha1, 3]GlcNAc)1 or its isomer sialyl-Lewis a (NeuAcalpha2, 3Galbeta1, 3[Fucalpha1,4]GlcNAc). We examined the effect of expressing the H-type alpha(1,2)-fucosyltransferase or the alpha(2, 6)-sialyltransferase on the synthesis of sialyl-Lewis x by alpha(1, 3)fucosyltransferase. We found that H-type alpha(1, 2)-fucosyltransferase but not alpha(2,6)-sialyltransferase, strongly inhibited sialyl-Lewis x expression and E-selectin adhesion. We assume that H-type alpha(1,2)-fucosyltransferase competes with the endogenous alpha(2,3)-sialyltransferase for the N-acetyllactosamine structures assigned to further serve as acceptors for alpha(1, 3)fucosyltransferase.

The formation of the oncofetal J28 glycotope involves core-2 beta6-N-acetylglucosaminyltransferase and alpha3/4-fucosyltransferase activities.

The feto-acinar pancreatic protein or FAPP, the oncofetal glycoisoform of bile salt-dependent lipase (BSDL), is characterized by the presence of the J28 glycotope recognized by mAbJ28. This fucosylated epitope is carried out by the O-linked glycans of the C-terminal mucin-like region of BSDL. This glycotope is expressed by human tumoral pancreatic tissues and by human pancreatic tumoral cell lines such as SOJ-6 and BxPC-3 cells. However, it is not expressed by the normal human pancreatic tissues and by MiaPaCa-2 and Panc-1 cells. Due to the presence of many putative sites for O-glycosylation on FAPP and BSDL, the structure of the J28 glycotope cannot be attained by classical physical methods. In the first part of the present study, we have determined which glycosyltransferases were differently expressed in pancreatic tumoral cell lines compared to normal tissues, focusing in part on fucosyltransferases (Fuc-T) and core-2 beta6-N-acetylglucosaminyltransferase (Core2GlcNAc-T). Our data suggested that alpha2-Fuc-T activity was decreased in the four cell lines tested (SOJ-6, BxPC-3, MiaPaCa-2, and Panc-1). The alpha(1-3) and alpha(1-4) fucosylations were decreased in tumor cells that do not express the J28 glycotope whereas alpha4-Fuc-T and Core2GlcNAc-T activities were significantly increased in SOJ-6 cells which best expressed the J28 glycotope. Therefore, we wished to gain information about glycosyltransferases involved in the building of this structure by transfecting the cDNA encoding the mucin-like region of BSDL in CHO-K1 also expressing Core2GlcNAc-T and/or FUT3 and/or FUT7 activities. These CHO-K1 cells have been previously transfected with the cDNA encoding Core2GlcNAc-T and/or FUT3 and/or FUT7. Data indicated that the C-terminal peptide of BSDL (Cter) produced by those cells did not carry out the J28 glycotope unless Core2GlcNAc-T activity is present. Further transfection with FUT3 cDNA, increased the antibody recognition. Nevertheless, transfection with FUT3 or FUT7 alone did not generate the formation of the J28 glycotope on the C-terminal peptide. Furthermore, the Cter peptide produced by CHO-K1 cells expressing Core2GlcNAc-T was more reactive to the mAbJ28 after in vitro fucosylation with the recombinant soluble form of FUT3. These data suggested that the J28 glycotope encompasses structures initiated by Core2GlcNAc-T and further fucosylated by alpha3/4-Fuc-T such as FUT3, likely on GlcNAc residues.

Green fluorescent protein-based system for analysis of E-selectin-mediated adhesion.

Numerous cell-based or cell-free systems for study of selectin adhesion use radiolabeled tracers. However, in addition to handling problems associated with the use of radioisotopes, these assays have difficulty relating a number of counts to a number of adherent cells. Here, we describe an assay that uses the natural fluorescence of the green fluorescent protein (GFP) to measure binding of cells to E-selectin. We elaborated an adhesion system composed of a cell monolayer expressing E-selectin ligand to which monodispersed fluorescent Chinese hamster ovary (CHO) cells expressing E-selectin are added. Due to GFP autofluorescence, adhered cells can be easily distinguished from cell monolayers by fluorescence microscopy, and adhesion can be measured by cytofluorometry. We applied this GFP-based adhesion assay to measure the adherence of a pancreatic tumor cell line and found that the binding parameters of these cells satisfy a number of E-selectin-specific criteria.

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.

Fucosyltransferase activities in human pancreatic tissue: comparative study between cancer tissues and established tumoral cell lines.

Human pancreatic cancer is characterized by an alteration in fucose-containing surface blood group antigens such as H antigen, Lewis b, Lewis y, and sialyl-Lewis. These carbohydrate determinants can be synthesized by sequential action of alpha(2,3) sialyltransferases or alpha(1,2) fucosyltransferases (Fuc-T) and alpha(1,3/1,4) fucosyltransferases on (poly)N-acetyllactosamine chains. Therefore, the expression and the function of seven fucosyltransferases were investigated in normal and cancer pancreatic tissues and in four pancreatic carcinoma cell lines. Transcripts of FUT1, FUT2, FUT3, FUT4, FUT5, and FUT7 were detected by RT-PCR in carcinoma cell lines as well as in normal and tumoral tissues. Interestingly, the FUT6 message was only detected in tumoral tissues. Analysis of the acceptor substrate specificity for fucosyltransferases indicated that alpha(1,2) Fuc-T, alpha(1,3) Fuc-T, and alpha(1,4) Fuc-T were expressed in microsome preparations of all tissues as demonstrated by fucose incorporation into phenyl beta-d-galactoside, 2'-fucosyllactose, N-acetyllactosamine, 3'-sialyl-N-acetyllactosamine, and lacto-N-biose. However, these fucosyltransferase activities varied between tissues. A substantial decrease of alpha(1,2) Fuc-T activity was observed in tumoral tissues and cell lines compared to normal tissues. Conversely, the activity of alpha(1,4) Fuc-T, which generates Lewis a and sialyl-Lewis a structures, and that of alpha(1,3) Fuc-T, able to generate a lactodifucotetraose structure, were very important in SOJ-6 and BxPC-3 cell lines. These increases correlated with an enhanced expression of Lewis a, sialyl-Lewis a, and Lewis y on the cell surface. The activity of alpha(1,3) Fuc-T, which participates in the synthesis of the sialyl-Lewis x structure, was not significantly modified in cell lines compared to normal tissues. However, the sialyl-Lewis x antigen was expressed preferentially on the surface of SOJ-6 and BxPC-3 cell lines but was not detected on Panc-1 and MiaPaca-2 cell lines suggesting that several alpha(1,3) Fuc-T might be involved in sialyl-Lewis x synthesis.

Altered Golgi localization of core 2 beta-1,6-N-acetylglucosaminyltransferase leads to decreased synthesis of branched O-glycans.

Mucin type O-glycans with core 2 branches are distinct from nonbranched O-glycans, and the amount of core 2 branched O-glycans changes dramatically during T cell differentiation. This oligosaccharide is synthesized only when core 2 beta-1, 6-N-acetylglucosaminyltransferase (C2GnT) is present, and the expression of this glycosyltransferase is highly regulated. To understand how O-glycan synthesis is regulated by the orderly appearance of glycosyltransferases that form core 2 branched O-glycans, the subcellular localization of C2GnT was determined by using antibodies generated that are specific to C2GnT. The studies using confocal light microscopy demonstrated that C2GnT was localized mainly in cis to medial-cisternae of the Golgi. We then converted C2GnT to a trans-Golgi enzyme by replacing its Golgi retention signal with that of alpha-2,6-sialyltransferase, which resides in trans-Golgi. Chinese hamster ovary cells expressing wild type C2GnT and the chimeric C2GnT were then subjected to oligosaccharide analysis. The results obtained clearly indicate that the conversion of C2GnT into a trans-Golgi enzyme resulted in a substantial decrease of core 2 branched oligosaccharides. These results, taken together, strongly suggest that the predominance of core 2 branched oligosaccharides in those cells expressing C2GnT is due to the fact that C2GnT is located earlier in the Golgi than alpha-2,3-sialyltransferase that competes with C2GnT for the common substrate. Furthermore, alteration of Golgi localization renders the chimeric C2GnT much less efficient in synthesizing core 2 branched oligosaccharides, indicating the critical role of orderly subcellular localization of glycosyltransferases.

Role of alpha v beta 5 and alpha v beta 6 integrin glycosylation in the adhesion of a colonic adenocarcinoma cell line (HT29-D4).

We have previously characterized the expression of the alpha nu beta 5 and alpha nu beta 6 integrins as major receptors for the human colonic adenocarcinoma cell line (HT29-D4), on vitronectin and fibronectin, respectively [Lehmann et al. (1994): Cancer Res 54:2102-2107]. In the present work we investigated the glycosylation role of these integrins in their adhesive functions. To this end, we used glycohydrolases to show that cell surface integrins were N-glycosylated and sialylated, and that only the alpha v subunit carried some immature oligosaccharide side chains. To alter the glycosylation state of the cell surface alpha v beta 5 and alpha v beta 6 integrins, we used two oligosaccharide-processing inhibitors: 1-deoxymannojirimycin (dMNJ) and tunicamycin (TM). Following treatment of HT29-D4 cells with dMNJ, cell surface alpha v beta 5 and alpha v beta 6 carried only high-mannose-type sugar chains, while TM-treated cells expressed de-N-glycosylated integrins. Neither alpha/beta heterodimers assembly nor cell surface expression were impaired in the presence of the drugs. Finally, we established that adhesion of dMNJ- or TM-treated cells was altered on both vitronectin and fibronectin substrata, whereas the adhesion of these cells on laminin or collagen type I was virtually unchanged.

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.

Structural and functional analysis of the human vasoactive intestinal peptide receptor glycosylation. Alteration of receptor function by wheat germ agglutinin.

The vasoactive intestinal peptide (VIP) receptor from the human melanoma cell line IGR39 has been shown to be a 60-kDa glycoprotein. Using serial lectin affinity chromatography, as well as specific glycosidases, we demonstrate that VIP receptor-linked carbohydrates are predominantly tri- or tetraantennary sialylated N-linked oligosaccharides, 27% of which are fucosylated, and some may have terminal galactose residues. Treatment of 125I-VIP receptor complexes with peptide-N4-(N-acetyl-beta-D-glucosaminyl)asparagine amidase revealed the presence of at least three N-linked carbohydrate chains/receptor polypeptide. To investigate the functional role of the carbohydrate moiety, 125I-VIP binding to IGR39 cell membranes was tested in the presence of soluble lectins. Among the lectins tested, only wheat germ agglutinin (WGA) was found to markedly inhibit VIP binding in a dose-dependent manner. Binding data indicated that the presence of the lectin led to a 3-fold increase in Kd value, from 0.15 to 0.44 nM, without any change in the number of available binding sites. The potent inhibitor of WGA binding, N,N',N"-triacetylchitotriose, completely reversed the effect of the lectin. On the other hand, VIP binding inhibition persisted even after neuraminidase treatment, suggesting that sialic acids were not directly involved. Furthermore, WGA inhibition was not abolished although most, if not all, VIP receptor oligosaccharides were converted to high mannose type structures by treating IGR39 cells with deoxymannojirimycin. Finally, whereas the pharmacological profile of VIP receptor was virtually identical, the presence of WGA greatly reduced the VIP-stimulated cAMP in IGR39 cells, indicating that the lectin alters the ability of the receptor to interact with the adenylate cyclase system.

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.

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.

Solubilization of the active vasoactive intestinal peptide receptor from human colonic adenocarcinoma cells.

The non-ionic detergent n-octyl-beta-D-glucopyranoside was used to solubilize the VIP (vasoactive intestinal peptide) receptor from human colonic adenocarcinoma cell line HT29-D4. The binding of monoiodinated 125I-VIP to the solubilized receptor was specific, time-dependent, and reversible. Scatchard analysis of data obtained from competitive displacement of monoiodinated 125I-VIP by native VIP suggested the presence of two classes of VIP binding sites with Kd values of 0.32 and 46.7 nM. The binding capacities of these two classes were 1.7 x 10(10) and 30.2 x 10(10) sites/mg of proteins, respectively. The solubilized receptor retained the specificity of the human VIP receptor towards the peptides of the VIP/secretin/glucagon family. The order of potency in inhibiting monoiodinated 125I-VIP binding was VIP (IC50 = 1.0 x 10(-9) M) much greater than peptide histidine methionine amide (IC50 = 10(-7) M) greater than growth hormone-releasing factor (IC50 = 3 x 10(-7) M) greater than secretin (IC50 greater than 10(-6) M); glucagon had no effect on VIP binding. The reducing agent dithiothreitol inhibited in a dose-dependent manner the binding of 125I-VIP. Covalent cross-linking experiments between the solubilized receptor and 125I-VIP showed that after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography two major and one minor polypeptides of Mr 67,000, 72,000, and 83,000 were specifically labeled. When analyzed by gel filtration, the n-octyl-beta-D-glucopyranoside-solubilized 125I-VIP-receptor complex was resolved into two major peaks with molecular mass in the range of 60-70 and 270-300 kDa. Thus, the soluble form of the VIP receptor was probably a multimeric complex in which disulfide bonds may play an important role to hold the receptor in an active configuration.

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.

HT 29, a model cell line: stimulation by the vasoactive intestinal peptide (VIP); VIP receptor structure and metabolism.

HT 29, a cell line derived from a human colonic adenocarcinoma, is highly responsive to the vasoactive intestinal peptide (VIP) as shown by a more than 100-fold intracellular cAMP increase (Ka = 0.3 nM), the stimulations of protein kinase A (Ka = 0.1 nM) and the low-Km cAMP phosphodiesterase (Ka = 40 nM). Remarkably, adenylate cyclase, cAMP-dependent kinase and cAMP-specific phosphodiesterase are activated in a sequential manner. Binding studies with [125I]-labeled VIP indicate a high affinity site with a Kd value (0.5 nM) close to the activation constant value (Ka) of the three enzymes. The molecular structure of the VIP receptor was studied by immunological and chemical approaches. A monoclonal antibody (mAb 109-10-16) which partially decreased the binding of VIP to its receptor allowed the characterization of Mr = 53,000 and Mr = 48-49,000 polypeptides. More precise identification of protein components of the VIP receptor resulted from covalent cross-linking on intact HT 29 cells by four bifunctional reagents: dithiobis-(succinimidyl propionate) and its non-cleavable analog disuccinimidyl suberate, the photoactivable azido phenyl glyoxal and dimethylpimelimidate. Analysis by SDS-polyacrylamide gel electrophoresis demonstrated a major band of Mr = 67,000 regardless of which cross-linker was used. The same band and an Mr = 49,000 species were found in experiments using a crude membrane fraction of HT 29 cells. Assuming one molecule of VIP (Mr = 3326) linked per polypeptide, these observations suggest that an Mr = 64,000 species belongs to the VIP specific plasma membrane receptor. This protein contains an Mr = 20,000 N-linked sialic acid rich oligosaccharidic moiety.(ABSTRACT TRUNCATED AT 250 WORDS)