Tyrosine phosphorylation of biliary glycoprotein, a cell adhesion molecule related to carcinoembryonic antigen

hCEACAM1-4L downregulates hDAF-associated signalling after being recognized by the Dr adhesin of diffusely adhering Escherichia coli

Human decay accelerating factor (hDAF, CD55) and members of the carcinoembryonic-antigen-related cell-adhesion molecules (hCEACAMs) family are recognized as receptors by Gram-negative, diffusely adhering Escherichia coli (DAEC) strains expressing Afa/Dr adhesins. We report here that hCEACAM1-4L has a key function in downregulating the protein tyrosine Src kinase associated with hDAF signalling. After infecting HeLa epithelial cells stably transfected with hCEACAM1-4L cDNA with Dr adhesin-positive E. coli, the amount of the pTyr(416)-active form of the Src protein decreased, whereas that of the pTyr(527)-inactive form of Src protein did not increase. This downregulation of the Src protein implies that part of the hCEACAM1-4L protein had been translocated into lipid rafts, the protein was phosphorylated at Tyr residues in the cytoplasmic domain, and it was physically associated with the protein tyrosine phosphatase, SHP-2. Finally, we found that the hCEACAM1-4L-associated SHP-2 was not phosphorylated and lacked phosphatase activity, suggesting that the downregulation of Src protein associated with hDAF signalling results from the absence of dephosphorylation of the pTyr(527)-inactive form necessary for Src kinase activation.

Characterization of recombinant soluble carcinoembryonic antigen cell adhesion molecule 1

Carcinoembryonic antigen cell adhesion molecule 1 (CEACAM1) is a type 1 transmembrane, homotypic cell adhesion protein expressed on epithelial and hematopoietic cells. CEACAM1 has four major isoforms with three or four immunoglobulin (Ig)-like ectodomains and either long or short cytoplasmic domains. In a 3D model of breast epithelial cell morphogenesis, CEACAM1 plays an essential role in lumen formation [J. Cell Sci. 112 (1999) 4193]. Two soluble ectodomain isoforms of CEACAM1 expressed in myeloma cells were immunologically active and highly glycosylated. The molecular weights of the 3-ecto- and 4-ectodomain isoforms were 90 and 110kDa, respectively, and monomers by sedimentation equilibrium centrifugation. Both isoforms were prolate ellipsoids with axial ratios of 6 for the 3-ecto- and 8 for 4-ectodomain isoforms, respectively, by size exclusion chromatography and analytical ultracentrifugation. Both isoforms caused a significant reduction in lumen formation when tested in the 3D model culture system.

Carcinoembryonic antigen cell adhesion molecule 1 directly associates with cytoskeleton proteins actin and tropomyosin

CEA cell adhesion molecule 1 (CEACAM1), a type 1 transmembrane and homotypic cell adhesion protein belonging to the carcinoembryonic antigen (CEA) gene family and expressed on epithelial cells, is alternatively spliced to produce four major isoforms with three or four Ig-like ectodomains and either long (CEACAM1-L) or short (CEACAM1-S) cytoplasmic domains. When murine MC38 (methylcholanthrene-induced adenocarcinoma 38) cells were transfected with human CEACAM1-L and stimulated with sodium pervanadate, actin was found to co-localize with CEACAM1-L at cell-cell boundaries but not in untreated cells. When CEACAM1-L was immunoprecipitated from pervanadate-treated MC38/CEACAM1-L cells and the associated proteins were analyzed by two-dimensional gel analysis and mass spectrometry, actin and tropomyosin, among other proteins, were identified. Whereas a glutathione S-transferase (GST) fusion protein containing the l-isoform (GST-Cyto-L) bound poorly to F-actin in a co-sedimentation assay, the S-isoform fusion protein (GST-Cyto-S) co-sedimented with F-actin, especially when incubated with G-actin during polymerization (K(D) = 7.0 microm). Both GST-Cyto-S and GST-Cyto-L fusion proteins bind G-actin and tropomyosin by surface plasmon resonance studies with binding constants of 0.7 x 10(-8) and 1.0 x 10(-7) m for GST-Cyto-L to G-actin and tropomyosin, respectively, and 3.1 x 10(-8) and 1.3 x 10(-7) m for GST-Cyto-S to G-actin and tropomyosin, respectively. Calmodulin or EDTA inhibited binding of the GST-Cyto-L fusion protein to G-actin, whereas calmodulin and G-actin, but not EDTA, stimulated binding to tropomyosin. A biotinylated 14-amino acid peptide derived from the juxtamembrane portion of the cytoplasmic domain of CEACAM1-L associated with both G-actin and tropomyosin with K(D) values of 1.3 x 10(-5) and 1.8 x 10(-5) m, respectively. These studies demonstrate the direct interaction of CEACAM1 isoforms with G-actin and tropomyosin and the direct interaction of CEACAM1-S with F-actin.

Synthetic peptides from the N-domains of CEACAMs activate neutrophils

Four members of the carcinoembryonic antigen family, CEACAM1, CEACAM8, CEACAM6 and CEACAM3, recognized by CD66a, CD66b, CD66c and CD66d monoclonal antibodies (mAb), respectively, are expressed on human neutrophils. CD66a, CD66b, CD66c and CD66d mAb binding to neutrophils triggers an activation signal that regulates the adhesive activity of CD11/CD18, resulting in an increase in neutrophil adhesion to human umbilical vein endothelial cells. Molecular modeling of CEACAM1 using IgG and CD4 as models has been performed, and three peptides from the N-terminal domain were found to increase neutrophil adhesion to human umbilical vein endothelial cell monolayers. The peptides were 14 amino acids in length and were predicted to be present at loops and turns between beta-sheets. To better understand the amino acid sequences critical for this biological activity, in the present study we examined the other neutrophil CEACAMs and the highly homologous CEACAM, CEA. Molecular modeling of the N-terminal domains of human CEACAM8, -6, -3 and CEA was performed. Twenty peptides, each 14 amino acids in length, that were homologous to the previously reported peptides from the N-domains of CEACAM1, were synthesized and tested for their ability to alter neutrophil adhesion. Only one new peptide, from the N-domain of CEA, was found to increase neutrophil adhesion, and this peptide differed from the corresponding CEACAM1 peptide by only a single conservative amino acid substitution. Importantly, minor amino acid differences between active and inactive homologous peptides suggest regions of these peptides that are critical for biological activity. The data suggest that the regions SMPF of peptide CD66a-1, QLFG of peptide CD66a-2 and NRQIV of peptide CD66a-3 are critical for the activities of these peptides, and for the native CEACAMs.

cis Interaction of the cell adhesion molecule CEACAM1 with integrin beta(3)

CEACAM1 is a cell adhesion molecule that has been implicated in a number of physiological processes (eg, tumor suppressor in epithelial tissues, potent angiogenic factor in microvessel formation, microbial receptor in human granulocytes and epithelial cells). The mechanism of CEACAM1 action is still largely unresolved but recent findings demonstrated that the cytoplasmic CEACAM1 domain is linked indirectly to the actin-based cytoskeleton. We have isolated integrin beta(3) as an associated protein using CEACAM1 tail affinity purification. This association depends on phosphorylation of Tyr-488 in the CEACAM1 cytoplasmic domain. Confocal laser scanning microscopy confirmed in vivo colocalization of both molecules in human granulocytes and epithelial cells. Furthermore, the concentrated colocalization at the tumor-stroma interface of invading melanoma masses suggests a functional role of CEACAM1-integrin beta(3) interaction in melanoma invasion. Moreover, colocalization of the two adhesion molecules is also found at the apical surface of glandular cells of pregnancy endometrium. Colocalization of CEACAM1 and integrin beta(3) at the transitional zone from proliferative to invasive extravillous trophoblast of the maternal-fetal interface supports a role for CEACAM1/integrin beta(3) complexes in cell invasion.

Synthetic peptides of CD66a stimulate neutrophil adhesion to endothelial cells

Four members of the carcinoembryonic Ag family, CD66a, CD66b, CD66c, and CD66d, are expressed on human neutrophils. CD66a, CD66b, CD66c, and CD66d Ab binding to the neutrophil surface triggers an activation signal that regulates the adhesive activity of CD11/CD18, resulting in an increase in neutrophil adhesion to HUVEC. To identify active sites on the CD66a Ag, molecular modeling was performed using IgG and CD4 as models, and 28 peptides of 14 aa in length were synthesized that were predicted to be present at loops and turns between beta-sheets. The peptides were tested for their ability to alter neutrophil adhesion to HUVEC. Three peptides, each from the N-terminal domain, increased neutrophil adhesion to HUVEC monolayers. This increase in neutrophil adhesion caused by CD66a peptides was associated with up-regulation of CD11/CD18 and down-regulation of CD62L on the neutrophil surface. Scrambled versions of these three peptides had no effect on neutrophil adhesion to the endothelial cells. The data suggest that peptide motifs from at least three regions of the N-terminal domain of CD66a are involved in the interaction of CD66a with other ligands and can initiate signal transduction in neutrophils.

Negative signaling in health and disease

Signal transduction induced by receptors can elicit intracellular biochemical events that either support or inhibit cell activation. Induction of the latter has been termed "negative signaling" and can be triggered by receptors on immune cells that are distinct from activating receptors while other growth-promoting receptors induce both positive and negative signaling events. Here, the biochemistry leading to cell activation or inhibition and induced by receptors on immune cells are reviewed. Furthermore, recent experimental evidence is reviewed that indicates an important contribution of negative signaling to the intracellular survival of infectious pathogens.

Murine hepatitis virus strain 3 induces the macrophage prothrombinase fgl-2 through p38 mitogen-activated protein kinase activation

The clinical syndrome of acute liver failure produced by fulminant viral hepatitis can be reproduced in mice by infection with murine hepatitis virus strain 3 (MHV-3). Although it is clear that MHV-3-induced hepatitis depends upon macrophage activation and the expression of a specific prothrombinase, fgl-2, the signaling pathways involved in virally stimulated cell activation are unclear. Since we had previously found that MHV-3 induces the tyrosine phosphorylation of cellular proteins, we investigated the roles of the mitogen-activated protein kinase (MAPK) proteins. In a series of Western blots, immunoprecipitation and in vitro kinase assay studies, we found that both the extracellular signal-related kinase (ERK) and p38 MAPK proteins are tyrosine-phosphorylated and activated following exposure of murine peritoneal exudative macrophages (PEM) to MHV-3. Although p38 phosphorylation and activity are induced soon after MHV-3 exposure, peaking by 1-5 min, ERK phosphorylation and activity increase more gradually, peaking at 20-30 min and gradually fading thereafter. Interestingly, whereas selective p38 inhibition with SB203580 (1-20 microM) abolished the virally stimulated induction of fgl-2 mRNA, protein, and functional activity, selective ERK inhibition with PD98059 (1-50 microM) limited fgl-2 functional activity but had little to no effect on fgl-2 mRNA or protein levels. Moreover, whereas inhibition of ERK had no effect on p38 activity, p38 inhibition consistently increased MHV-3-induced ERK activity. To ensure that these pathways were relevant in vivo, MHV-3 was injected intraperitoneally, and peritoneal exudative macrophages were collected. Again, MHV-3 exposure led to increased p38 and ERK tyrosine phosphorylation. These data argue that MHV-3 induces tightly interconnected ERK and p38 MAPK cascades in the macrophage both in vitro and in vivo. Although the ERK and p38 MAPK proteins have discordant effects at the level of fgl-2 expression, both converge at the level of its activity, suggesting that targeted MAPK inhibition may ultimately be useful in the modulation of viral hepatitis.

Biliary glycoprotein (CD66a), a cell adhesion molecule of the immunoglobulin superfamily, on human lymphocytes: structure, expression and involvement in T cell activation

The biliary glycoproteins (BGP or CD66a), a group of different splice variants of a single gene, are members of the carcinoembryonic antigen family and the immunoglobulin superfamily. Recently, we detected CD66a on IL-2 activated lymphocytes. In this study we characterized the structure and the expression pattern of BGP on human lymphocytes and investigated its role in T cell activation. Lymphocytes express 2 of the 13 known splice variants, i.e. BGPa and BGPb, which are glycosylated in a lymphocyte-specific manner. Both BGPa and BGPb have the long cytoplasmic tail, which contains two immunoreceptor tyrosine-based inhibitory motif (ITIM)-like motifs, but differ in their extracellular region containing 4 and 3 immunoglobulin-like domains, respectively. On PBL BGP is expressed in small amounts only on B cells and Th cells. Stimulation with IL-2 leads to a strong up-regulation of BGP by these cells, and induces de novo BGP expression on gammabeta T cells, CD8+ and CD56+ cells, but not on CD16+ lymphocytes. This up-regulation of BGP seems to be part of the physiological process of T cell activation, since stimulation with anti-CD3 mAb is sufficient to induce BGP up-regulation. Based on the presence of the two ITIM-like motifs, one may expect that BGP inhibits T cell activation, but surprisingly, engagement of BGP enhances the proliferation of anti-CD3-stimulated T cells.

The cell adhesion molecule C-CAM is a substrate for tissue transglutaminase

C-CAM, a ubiquitously expressed cell adhesion molecule belonging to the carcinoembryonic antigen family, appears as two co-expressed isoforms, C-CAM-L and C-CAM-S, with different cytoplasmic domains, that can form homodimers in epithelial cells. In addition, C-CAM-L has been found in large molecular weight forms suggesting posttranslational, covalent modification. Here we have investigated the possibility that the cytoplasmic domain of C-CAM-L can act as a transglutaminase substrate. Glutathione S-transferase fusion proteins of the cytoplasmic domains of rat and mouse C-CAM-L as well as free cytoplasmic domains, released by thrombin cleavage from the fusion proteins, were converted into covalent dimers by tissue transglutaminase. These results demonstrate that the cytoplasmic domains of rat and mouse C-CAM-L are substrates for tissue transglutaminase, and lend support to the notion that higher molecular weight forms of C-CAM-L are formed by transglutaminase modification.

Characterization of protein kinase C-mediated phosphorylation of the short cytoplasmic domain isoform of C-CAM

C-CAM is a ubiquitously expressed cell adhesion molecule belonging to the carcinoembryonic antigen family. Two co-expressed isoforms, C-CAM-L and C-CAM-S, are known, having different cytoplasmic domains both of which can be phosphorylated in vivo. Here we have characterized the PKC-mediated phosphorylation of the short cytoplasmic domain isoform, C-CAM-S. Phorbol myristyl acetate induced phosphorylation of C-CAM-S in transfected CHO cells. Using synthetic peptides and Edman degradation we identified Ser449 as the PKC-phosphorylated amino acid residue. Binding experiments with modified peptides indicated that this phosphorylation decreases the ability of the cytoplasmic domain of C-CAM-S to bind calmodulin.

Tyrosine residue in exon 14 of the cytoplasmic domain of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) regulates ligand binding specificity

Platelet/endothelial cell adhesion molecule (PECAM-1) is a cell adhesion molecule of the immunoglobulin superfamily that plays a role in a number of vascular processes including leukocyte transmigration through endothelium. The presence of a specific 19- amino acid exon within the cytoplasmic domain of PECAM-1 regulates the binding specificity of the molecule; specifically, isoforms containing exon 14 mediate heterophilic cell-cell aggregation while those variants missing exon 14 mediate homophilic cell-cell aggregation. To more precisely identify the region of exon 14 responsible for ligand specificity, a series of deletion mutants were created in which smaller regions of exon 14 were removed. After transfection into L cells, they were tested for their ability to mediate aggregation. For heterophilic aggregation to occur, a conserved 5-amino acid region (VYSEI in the murine sequence or VYSEV in the human sequence) in the mid-portion of the exon was required. A final construct, in which this tyrosine was mutated into a phenylalanine, aggregated in a homophilic manner when transfected into L cells. Inhibition of phosphatase activity by exposure of cells expressing wild type or mutant forms of PECAM-1 to sodium orthovanadate resulted in high levels of cytoplasmic tyrosine phosphorylation and led to a switch from heterophilic to homophilic aggregation. Our data thus indicate either loss of this tyrosine from exon 14 or its phosphorylation results in a change in ligand specificity from heterophilic to homophilic binding. Vascular cells could thus determine whether PECAM-1 functions as a heterophilic or homophilic adhesion molecule by processes such as alternative splicing or by regulation of the balance between tyrosine phosphorylation or dephosphorylation. Defining the conditions under which these changes occur will be important in understanding the biology of PECAM-1 in transmigration, angiogenesis, development, and other processes in which this molecule plays a role.

Evidence for regulated dimerization of cell-cell adhesion molecule (C-CAM) in epithelial cells

C-CAM is a Ca(2+)-independent cell adhesion molecule (CAM) belonging to the immunoglobulin superfamily. Addition of chemical cross-linkers to isolated rat liver plasma membranes, intact epithelial cells and purified preparations of C-CAM stabilized one major C-CAM-containing product whose apparent molecular mass was approximately twice that of the C-CAM monomer. The failure to detect additional proteins after cleavage of the cross-linked species demonstrated that C-CAM exists as non-covalently linked dimers both in solution and on the cell surface. Dimerization occurred to the same extent in adherent monolayers and in single cell populations, indicating that dimer formation was the result of cis-interactions within the membranes of individual cells. Using isoform-specific anti-peptide antibodies, both C-CAM1 and C-CAM2 were found to be involved in dimerization, forming predominantly homo-dimeric species. Both calmodulin and Ca2+ ionophore modulated the level of dimer formation, suggesting a role for regulated self-association in the functional activity of C-CAM.

CD66: role in the regulation of neutrophil effector function

Neutrophils express several heavily glycosylated carcinoembryonic antigen (CEA)-related glycoproteins (CD66 antigens) which have been implicated in adhesion to E-selectin and as receptors for the lectins galectin 3 and bacterial type-1 fimbriae. The role of the CD66 antigens in neutrophil effector function was examined using non-cross-reacting and cross-reacting domain-mapped CD66 monoclonal antibody (mAb), which recognize epitopes on biliary glycoprotein (BGP; CD66a), CEA gene family member 6 (CGM6; CD66b), nonspecific cross-reacting antigen 90 (NCA90; CD66c) or CGM1 (CD66d). We show that BGP-specific mAb which recognize an AB-domain epitope strongly augment adhesion to fibrinogen by an Fc receptor- and beta2 integrin-dependent mechanism. Co-ligation of BGP with the glycophosphatidylinositol (GPI)-anchored CGM6 and NCA90 also caused increased beta2 integrin-mediated adhesion, receptor clustering and priming of formyl-Met-Leu-Phe (fMLP)-induced oxidant production by neutrophils, but only a small change in expression of L-selectin and CR3 compared to the chemotactic peptide fMLP. Ligation of CGM6 or NCA90 alone did not cause activation of the neutrophil in any of the assays used and did not cause priming of fMLP-induced oxidant production even when a secondary cross-linking reagent was used. We propose that specific cross-linking of neutrophil BGP with CGM6 and NCA90 contributes significantly to the regulation of neutrophil function during neutrophil recruitment.

Role of interferon regulatory factor-1 in the induction of biliary glycoprotein (cell CAM-1) by interferon-gamma

Biliary glycoprotein (BGP), also known as C-CAM-1, has been shown to be down-regulated in colon and prostate tumors. Previously, we demonstrated that BGP mRNA is up-regulated by interferon-gamma (IFN-gamma) in colon cancer cell lines (Takahashi, H., Okai, Y., Paxton, R. J., Hefta, L. J. F., and Shively, J. E. (1993) Cancer Res. 53, 1612-1619). We now show that the BGP promoter contains an interferon-sensitive response element (ISRE) that is specifically protected in in vivo footprints. Interferon regulatory factor-1 (IRF-1) was identified as the ISRE-binding factor by electrophoretic mobility shift assays. The induction of IRF-1 mRNA by IFN-gamma in HT-29 cells reaches a maximum at 6 h and is superinduced by cycloheximide. Four mRNA species for BGP are induced by IFN-gamma, the major band of which is inhibited by cycloheximide. Transfection of HT-29 cells with an IRF-1 expression plasmid (pAct-1) transactivates a BGP promoter reporter gene containing wild-type (but not mutant) ISRE. Electrophoretic mobility shift assay analysis of a second footprint reveals the binding of Sp1, an Sp1-like protein, and upstream stimulatory factor. The Sp1-like complex was also induced by IFN-gamma treatment of HT-29 cells and may be a second point of transcriptional control for the BGP gene.

Biliary glycoprotein 1 expression during embryogenesis: correlation with events of epithelial differentiation, mesenchymal-epithelial interactions, absorption, and myogenesis

Biliary glycoprotein (Bgp1), a carcinoembryonic antigen-related family member of the immunoglobulin superfamily, is involved in normal and neoplastic events. Analysis of Bgp1 expression throughout post-implantation mouse embryogenesis using reverse transcription-polymerase chain reactions, immunostaining with anti-Bgp1 monoclonal antibodies, and in situ hybridization with specific Bgp1 cDNA fragments revealed that Bgp1 may be involved in a number of specific embryonic processes. Immunoblot analysis of Bgp1 deletion mutant proteins indicated that distinguishable epitopes of the molecule were preferentially identified by the three Bgp1 antibodies used in this study. This distinction is supported by our immunolocalization studies during mouse embryogenesis in which the three antibodies revealed specific patterns of Bgp1 expression. Bgp1 is not expressed in early post-implantation embryos (7.5 dpc), but is found in the placenta and extra-embryonic tissues (decidual endothelial cells, giant trophoblasts, yolk sac visceral endoderm, and endometrial glands) at this time. The primitive gut epithelium and surface ectoderm were the first embryonic tissues to express Bgp1. Significant Bgp1 expression was also observed later during epithelial-mesenchymal interactions (skin, meninges, lung, kidney, salivary glands, pancreas). A unique epitope of Bgp1, detectable by the monoclonal antibody CC1, was also associated with mesenchymal expression and was prominent during myogenesis (secondary myotube formation) at sites of terminal differentiation. These studies suggest multiple roles for isoforms and glycoforms of the Bgp1 proteins localized in specific sites during prenatal development.

Characterization and transcriptional activity of the mouse biliary glycoprotein 1 gene, a carcinoembryonic antigen-related gene

The mouse biliary glycoprotein 1 gene (bgp1) encodes several multifunctional glycoprotein isoforms. These glycoproteins represent members of the carcinoembryonic antigen (CEA) family which belongs to the immunoglobulin superfamily. The Bgp1 glycoproteins function as cell adhesion molecules and receptors for the mouse hepatitis viruses. In contrast to CEA, whose overexpression has been correlated with cancer progression, the human and mouse Bgp proteins are generally down-regulated upon tumor formation. In this study, we report on the mouse bgp1 gene organization and transcriptional activation. We have isolated phage and cosmid clones encompassing the entire bgp1 coding region. This gene consists of nine exons, some of which are subjected to alternative splicing producing a minimum of four splice variants. A comparison of the murine bgp1 proximal promoter with the human BGP and mouse cea10/bgp3 genes revealed sequence conservation of 66% and 95%, respectively. RNase protection assays and primer extension analyses indicated that the mouse bgp1 transcriptional start site is positioned 240 nucleotides upstream of the ATG translational initiation codon, which is 140 nucleotides further upstream than in any other CEA family member. The bgp1 promoter is transcriptionally active in reporter gene activation in vitro transfection studies and in vivo using a bgp1-containing cosmid clone. We identified three putative AP-2 or AP-2-like sites and an upstream stimulatory factor (USF) recognition sequence within the proximal mouse bgp1 promoter region at positions similar to those used by the human BGP promoter region. These data suggest that the regulation of the mouse and human BGP genes may follow some common spatial and temporal expression. Interestingly, the bgp1 proximal promoter and coding region are also well conserved throughout evolution.

Transcriptional control of the human biliary glycoprotein gene, a CEA gene family member down-regulated in colorectal carcinomas

Biliary glycoprotein (BGP) isoantigens are derived by alternative splicing from a single gene and are the human homologs of rat C-CAM and the mouse Bgp species. These glycoproteins represent a family of cell-adhesion molecules. The mouse Bgp isoforms also act as receptors for the hepatitis viral capsid-protein. BGP is a member of the carcinoembryonic antigen (CEA) gene family, which belongs to the immunoglobulin supergene family, yet it displays restricted expression patterns and unique functions. Since the loss or reduced expression of BGP is associated with human colorectal carcinomas, the elements in its upstream regulatory region were analyzed. A cluster of transcriptional initiation sites and the minimal promoter, located within 150 bp upstream of the major transcriptional start site, were active in human colon carcinoma and hepatoma cells. Unlike the CEA gene, BGP gene transcription was not modulated by a silencer region; repetitive elements in the BGP upstream region were not involved in activation or repression. Footprinting experiments identified two cis-acting elements and mobility-shift assays demonstrated that these elements bound several transcription factors, among them, USF, HNF-4 and an AP-2-like factor. In cotransfection experiments, both the USF and HNF-4 transcription factors transactivate the BGP gene promoter and compete for the same regulatory element. The Sp1 transcription factor, shown to be involved in CEA gene transcriptional regulation, does not bind to the BGP gene promoter. We, therefore, propose that the relative distributions and interactions of these transcription factors mediate distinct transcriptional regulation of the BGP gene in colon and liver; this regulation could be distorted during the oncogenic process.

Bgp2, a new member of the carcinoembryonic antigen-related gene family, encodes an alternative receptor for mouse hepatitis viruses

Murine coronaviruses such as mouse hepatitis virus (MHV) infect mouse cells via cellular receptors that are isoforms of biliary glycoprotein (Bgp) of the carcinoembryonic antigen gene family (G. S. Dveksler, C. W. Dieffenbach, C. B. Cardellichio, K. McCuaig, M. N. Pensiero, G.-S. Jiang, N. Beauchemin, and K. V. Holmes, J. Virol. 67:1-8, 1993). The Bgp isoforms are generated through alternative splicing of the mouse Bgp1 gene that has two allelic forms called MHVR (or mmCGM1), expressed in MHV-susceptible mouse strains, and mmCGM2, expressed in SJL/J mice, which are resistant to MHV. We here report the cloning and characterization of a new Bgp-related gene designated Bgp2. The Bgp2 cDNA allowed the prediction of a 271-amino-acid glycoprotein with two immunoglobulin domains, a transmembrane, and a putative cytoplasmic tail. There is considerable divergence in the amino acid sequences of the N-terminal domains of the proteins coded by the Bgp1 gene from that of the Bgp2-encoded protein. RNase protection assays and RNA PCR showed that Bgp2 was expressed in BALB/c kidney, colon, and brain tissue, in SJL/J colon and liver tissue, in BALB/c and CD1 spleen tissue, in C3H macrophages, and in mouse rectal carcinoma CMT-93 cells. When Bgp2-transfected hamster cells were challenged with MHV-A59, MHV-JHM, or MHV-3, the Bgp2-encoded protein served as a functional MHV receptor, although with a lower efficiency than that of the MHVR glycoprotein. The Bgp2-mediated virus infection could not be inhibited by monoclonal antibody CC1 that is specific for the N-terminal domain of MHVR. Although CMT-93 cells express both MHVR and Bgp2, infection with the three strains of MHV was blocked by pretreatment with monoclonal antibody CC1, suggesting that MHVR was the only functional receptor in these cells. Thus, a novel murine Bgp gene has been identified that can be coexpressed in inbred mice with the Bgp1 glycoproteins and that can serve as a receptor for MHV strains when expressed in transfected hamster cells.

Differential regulation of C-CAM isoforms in epithelial cells

C-CAM is a Ca(2+)-independent cell adhesion molecule (CAM) that mediates intercellular adhesion of isolated rat hepatocytes. It is widely distributed in epithelia, where its presence both at lateral cell borders and on apical cell surfaces suggests that it may have diverse biological functions. Two major isoforms, C-CAM1 and C-CAM2, which differ in the lengths of their cytoplasmic domains, have been identified. The lack of suitable in vitro systems has so far prevented a detailed study of the physiological role of C-CAM in epithelia. We now report on the identification, biochemical characterization and functional analysis of C-CAM isoforms in the established epithelial cell line NBT II, derived from a chemically induced carcinoma of rat bladder. C-CAM in NBT II cells is a 110–115 kDa cell surface glycoprotein located predominantly at sites of cell-cell contact but also present on the apical cell surface. Northern blotting analysis revealed the presence of both C-CAM1 and C-CAM2, with the major transcripts for both isoforms present within the 4.0 kb size range. The dissociation of NBT II cell colonies by anti-C-CAM antibodies indicated that at least one function of C-CAM in these cells is to mediate intercellular adhesion. The maintenance of extensive cell-cell contacts and the expression of C-CAM at the contact sites in cells grown in low Ca2+ medium suggested that, like its counterpart in hepatocytes, C-CAM in NBT II cells may be a Ca(2+)-independent cell-cell adhesion molecule. The co-localization and coordinate reorganization of both C-CAM and actin by anti-C-CAM antibodies indicated that these two proteins were associated and suggested that interactions with the cytoskeleton may be important for the regulation of C-CAM function. The specific upregulation of C-CAM1 in cells induced to undergo epithelial to mesenchymal-like transitions (EMT) by the serum substitute Ultroser G suggested that C-CAM isoforms are important modulators of the adhesive properties of these cells.

Human carcinoembryonic antigen, an intercellular adhesion molecule, blocks fusion and differentiation of rat myoblasts

Human carcinoembryonic antigen (CEA), a widely used tumor marker, is a member of a family of cell surface glycoproteins that are overexpressed in many carcinomas. CEA has been shown to function in vitro as a homotypic intercellular adhesion molecule. This correlation of overproduction of an adhesion molecule with neoplastic transformation provoked a test of the effect of CEA on cell differentiation. Using stable CEA transfectants of the rat L6 myoblast cell line as a model system of differentiation, we show that fusion into myotubes and, in fact, the entire molecular program of differentiation, including creatine phosphokinase upregulation, myogenin upregulation, and beta-actin downregulation are completely abrogated by the ectopic expression of CEA. The blocking of the upregulation of myogenin, a transcriptional regulator responsible for the execution of the entire myogenic differentiation program, indicates that CEA expression intercepts the process at a very early stage. The adhesion function of CEA is essential for this effect since an adhesion-defective N domain deletion mutant of CEA was ineffective in blocking fusion and CEA transfectants treated with adhesion-blocking peptides fused normally. Furthermore, CEA transfectants maintain their high division potential, whereas control transfectants lose division potential with differentiation similarly to the parental cell line. Thus the expression of functional CEA on the surface of cells can block terminal differentiation and maintain proliferative potential.

Phosphorylation and disruption of intermediate filament proteins in oligodendrocyte precursor cultures treated with calyculin A

Treatment of primary cultures of oligodendrocyte precursors with calyculin A, a potent inhibitor of protein phosphatases 1 and 2A, caused the phosphorylation of two intermediate filament components, nestin and vimentin. Phosphoamino acid analysis demonstrated that phosphorylation took place mainly on serine and to a lesser extent on threonine residues. In addition, calyculin A treatment caused a shift in the distribution of the two proteins from the Triton-X-100 insoluble fraction to the detergent soluble fraction as demonstrated by immunoblotting. This redistribution, which was evident within 15 min after treatment and was nearly completed by 90 min, was accompanied by a disruption of the intermediate filament network. Thus, both nestin and vimentin retracted from the cytoplasmic processes to form a large perikaryal ring as shown by immunocytochemical analysis. Both morphological and biochemical changes were reversed 2-5 hr after removal of calyculin A from the culture medium.

Expression of the Bgp gene and characterization of mouse colon biliary glycoprotein isoforms

The biliary glycoprotein (BGP)-encoding gene is a member of the human carcinoembryonic antigen (CEA) gene family. We have now cloned several mouse Bgp cDNAs from an outbred CDR-1 mouse colon cDNA library, as well as by reverse transcription-PCR amplification of colon RNA. The distinguishing features of the deduced Bgp protein isoforms are found in the two divergent N-terminal domains, the highly conserved internal C2-set immunoglobulin domains, and an intracytoplasmic domain of either 10 or 73 amino acids (aa). The cDNA structures suggest that these mRNAs are produced through alternative splicing of a Bgp gene and the usage of multiple transcriptional terminators. The Bgp deduced aa sequences are highly homologous to several well characterized rat hepatocyte proteins such as the cell CAM105/ecto-ATPase/pp120/HA4 proteins. Oligodeoxyribonucleotide probes representing the various cDNA isoform domains revealed predominant transcripts of 1.8, 3.1 and 4.0 kb on Northern analyses of mouse colon RNA; some of these bands are actually composed of several co-migrating transcripts. The transcripts encoding the long intracytoplasmic-tailed Bgp proteins are expressed at one-tenth the relative abundance of the shorter-tailed species. We have previously demonstrated that several mouse Bgp cDNAs, when transfected into eukaryotic cells, express BGP proteins at the cell surface and function in vitro as cell adhesion molecules, much like their human and rat counterparts. The expression of the many Bgp isoforms at the surface of epithelial cells, such as colon, suggests that these proteins play a determinant role, through self- or heterologous contact, in renewal and/or differentiation of their epithelia.

Genomic organization, splice variants and expression of CGM1, a CD66-related member of the carcinoembryonic antigen gene family

The tumor marker carcinoembryonic antigen (CEA) belongs to a family of proteins which are composed of one immunoglobulin variable domain and a varying number of immunoglobulin constant-like domains. Most of the membrane-bound members, which are anchored either by a glycosylphosphatidylinositol moiety or a transmembrane domain, have been shown to convey cell adhesion in vitro. Here we describe two splice variants of CGM1, a transmembrane member of the CEA family without immunoglobulin constant-like domains. CGM1a and CGM1c contain cytoplasmic domains of 71 and 31 amino acids, respectively. The cytoplasmic region of CGM1a is encoded by four exons (Cyt1-Cyt4). Differential splicing of the Cyt1 exon (53 bp) leads to the formation of CGM1c. The presence or absence of potential protein kinase phosphorylation sites in the cytoplasmic domains and a sequence consensus motif involved in signal transduction in multichain immune recognition receptors indicates that this splice event is of functional importance. CGM1a mRNA, the predominant CGM1 transcript, was found in the granulocytic lineage, but not in monocytes, lymphocytes nor in a number of tumors derived from all three germ layers. Weak staining using monoclonal antibodies Tu2 and 73 in fluorescence-activated cell scan analyses indicate low concentrations of CGM1 protein on the surface of granulocytes. The CGM1 protein is also recognized by CD66 antibodies. Therefore, the granulocyte-specific CD66 epitope is present on at least four CEA family members: CGM1, CEA, NCA-50/90 and NCA-160.

Antiserum to carcinoembryonic antigen recognizes a phosphotyrosine-containing protein in human colon cancer cell lines

Members of the carcinoembryonic antigen (CEA) family include CEA, non-specific cross reacting antigen (NCA), and biliary glycoprotein (BGP), and appear to function as cell adhesion molecules. Immunoprecipitation and subsequent gel electrophoresis of proteins from several colon cancer cell lines labeled with [gamma-32P]ATP, under conditions designed to detect ecto-kinase-catalyzed phosphorylation of cellular proteins, revealed that polyclonal anti-CEA antiserum recognized a 175-190 kDa phosphoprotein on the surface of colon cancer cells. The ability to detect this phosphoprotein did not correlate with CEA production, and immunoprecipitation studies suggested that the phosphoprotein is BGP. Phosphoamino acid analysis of the 175-190 kDa protein showed that it contained predominantly phosphotyrosine.

Human biliary glycoprotein gene: characterization of a family of novel alternatively spliced RNAs and their expressed proteins

Eight different human biliary glycoprotein (BGP) isoantigens, structurally related members of the carcinoembryonic antigen family, CD66/67 family, and immunoglobulin superfamily, are derived by alternative splicing from a single genomic transcription unit. Novel BGP isoforms have been identified by polymerase chain reaction amplification and by DNA sequencing of amplified cDNA segments. In addition to verifying previously documented BGPs, we describe four new forms, two of which have unusual nonimmunoglobulin exons contributed by inverted Alu repeats. Determination of the genomic DNA sequence encompassing most of the known extracellular and intracellular domains demonstrates that the translatable Alu-like sequences are encoded in bona fide exons. The third novel BGP isoform contains none of the extracellular disulfide-linked immunoglobulin-like domains typical of these molecules but retains N-terminal and intracellular domains, suggesting distinct functions for N-terminal versus other disulfide-linked domains. cDNAs coding for each identified isoform have been transfected into COS7 monkey cells, and the resulting polypeptides are heavily N glycosylated but can be deglycosylated to their expected primary sizes. Many of these deglycosylated forms can be correlated with unique patterns of BGP expression in different cell lines, while in granulocytes, some previously undescribed or alternatively modified forms may predominate. The BGP family represents a potentially large but unknown source of functional diversity among cells of epithelial and hematopoietic origin. The availability of a defined set of expressed of BGP cDNAs should permit critical definition of their function.

Human biliary glycoprotein gene: characterization of a family of novel alternatively spliced RNAs and their expressed proteins

Eight different human biliary glycoprotein (BGP) isoantigens, structurally related members of the carcinoembryonic antigen family, CD66/67 family, and immunoglobulin superfamily, are derived by alternative splicing from a single genomic transcription unit. Novel BGP isoforms have been identified by polymerase chain reaction amplification and by DNA sequencing of amplified cDNA segments. In addition to verifying previously documented BGPs, we describe four new forms, two of which have unusual nonimmunoglobulin exons contributed by inverted Alu repeats. Determination of the genomic DNA sequence encompassing most of the known extracellular and intracellular domains demonstrates that the translatable Alu-like sequences are encoded in bona fide exons. The third novel BGP isoform contains none of the extracellular disulfide-linked immunoglobulin-like domains typical of these molecules but retains N-terminal and intracellular domains, suggesting distinct functions for N-terminal versus other disulfide-linked domains. cDNAs coding for each identified isoform have been transfected into COS7 monkey cells, and the resulting polypeptides are heavily N glycosylated but can be deglycosylated to their expected primary sizes. Many of these deglycosylated forms can be correlated with unique patterns of BGP expression in different cell lines, while in granulocytes, some previously undescribed or alternatively modified forms may predominate. The BGP family represents a potentially large but unknown source of functional diversity among cells of epithelial and hematopoietic origin. The availability of a defined set of expressed of BGP cDNAs should permit critical definition of their function.

Homotypic and heterotypic Ca(++)-independent cell adhesion activities of biliary glycoprotein, a member of carcinoembryonic antigen family, expressed on CHO cell surface

Homotypic and heterotypic cell adhesion activities of a carcinoembryonic antigen (CEA) family member, biliary glycoprotein a (BGPa), have been examined. CHO cells transfected with the cDNA for BGPa, CEA, non-specific cross-reacting antigen (NCA) and CGM6 have been used. The BGPa producers showed both homotypic and heterotypic adhesion to CEA and NCA producers. However, they hardly adhered to CGM6 producers. Calcium ion was not required for BGPa-mediated homotypic and heterotypic cell adhesion as well as for the adhesions of other members of CEA family. The results strongly suggested that BGPa may play some important roles through Ca(++)-independent cell adhesion activities.