The CEA Family: A System in Transitional Evolution?

The CEA family consists of two structurally and functionally distinct subgroups; the group including CEA, NCA and CGM-6 which are cell surface-bound by phosphatidylinositol (PI) linkages, and the group of BGP splice variants which have transmembrane and cytoplasmic domains. Although all CEA family members mediate intercellular adhesion in vitro, the PI-linked group show Ca++ and temperature independent adhesion whereas the BGP group show rapidly reversible Ca++ and temperature dependent adhesion. From the close alignment in cDNA nucleotide sequences between family members and between repeated domains in one family member, it is apparent that the CEA family is now rapidly evolving; in fact, analogs of only the transmembrane BGP group have been found so far in the mouse. The addition of a new group of potent adhesion molecules to complex species at some time after the rodent radiation has strong evolutional implications, which are discussed.

Adherent-invasive Escherichia coli induce claudin-2 expression and barrier defect in CEABAC10 mice and Crohn's disease patients

BACKGROUND

Abnormal expression of CEACAM6 observed on the ileal epithelium in Crohn's disease (CD) patients allows adherent-invasive Escherichia coli (AIEC) to colonize gut mucosa. Since intestinal permeability is significantly increased in CD patients, we aimed at investigating whether and how AIEC alter barrier function.

METHODS

Tissue microarray was performed on ileal biopsies from CD patients in quiescent and active phases. CEABAC10 or wildtype mice were orally challenged with 10(9) bacteria. Intestinal permeability was assessed by measuring 4 kDa dextran-FITC flux in serum, barrier integrity was analyzed using biotin tracer experiment, and claudin-2 protein immunostaining. Bacterial translocation was analyzed in Ussing chambers.

RESULTS

Pore-forming tight junction protein claudin-2 is strongly expressed in the ileum of 51% patients in quiescent phase and in 49% of the patients with active CD. Infection of CEABAC10 transgenic mice expressing human CEACAMs with AIEC, but not with nonpathogenic E. coli, led to a significant 3.0-fold increase in intestinal permeability and to disruption of mucosal integrity in a type 1 pili-dependent mechanism. This is consistent with the claudin-2 abnormal expression at the plasma membrane of intestinal epithelial cells observed in AIEC-infected CEABAC10 mice. AIEC bacteria were able to translocate through CEABAC10 intestinal mucosa.

CONCLUSIONS

These findings strongly support the hypothesis that AIEC type 1 pili-mediated interaction with CEACAM6 abnormally expressed in the quiescent phase of CD may disrupt intestinal barrier integrity before the onset of inflammation. Thus, therapeutic targeting claudin-2 induced by AIEC infection could be a new clinical strategy for preserving intestinal barrier function in CD patients.

Crohn's disease adherent-invasive Escherichia coli colonize and induce strong gut inflammation in transgenic mice expressing human CEACAM

Abnormal expression of CEACAM6 is observed at the apical surface of the ileal epithelium in Crohn's disease (CD) patients, and CD ileal lesions are colonized by pathogenic adherent-invasive Escherichia coli (AIEC). We investigated the ability of AIEC reference strain LF82 to colonize the intestinal mucosa and to induce inflammation in CEABAC10 transgenic mice expressing human CEACAMs. AIEC LF82 virulent bacteria, but not nonpathogenic E. coli K-12, were able to persist in the gut of CEABAC10 transgenic mice and to induce severe colitis with reduced survival rate, marked weight loss, increased rectal bleeding, presence of erosive lesions, mucosal inflammation, and increased proinflammatory cytokine expression. The colitis depended on type 1 pili expression by AIEC bacteria and on intestinal CEACAM expression because no sign of colitis was observed in transgenic mice infected with type 1 pili–negative LF82-ΔfimH isogenic mutant or in wild-type mice infected with AIEC LF82 bacteria. These findings strongly support the hypothesis that in CD patients having an abnormal intestinal expression of CEACAM6, AIEC bacteria via type 1 pili expression can colonize the intestinal mucosa and induce gut inflammation. Thus, targeting AIEC adhesion to gut mucosa represents a new strategy for clinicians to prevent and/or to treat ileal CD.

Colorectal hyperplasia and dysplasia due to human carcinoembryonic antigen (CEA) family member expression in transgenic mice

CEA and CEACAM6 are immunoglobulin family intercellular adhesion molecules that are up-regulated without structural mutations in approximately 70% of human cancers. Results in in vitro systems showing tumorigenic effects for these molecules suggest that this correlation could indicate an instrumental role in tumorigenesis. To test whether this applies in vivo, transgenic mice harboring 187 kb of the human genome containing four CEA family member genes including the CEA and CEACAM6 genes were created and their copy numbers increased by mating until colonocyte expression levels reached levels seen in human colorectal carcinomas. The colonocyte surface level of integrin alpha5 and the activation of AKT increased progressively with the expression levels of CEA/CEACAM6. Colonic crypts showed a progressive increase in colonocyte proliferation, an increase in crypt fission, and a strong inhibition of both differentiation and anoikis/apoptosis. All transgenic mice showed massively enlarged colons comprising a continuous mosaic of severe hyperplasia, dysplasia and serrated adenomatous morphology. These results suggest that up-regulated non-mutated adhesion molecules could have a significant instrumental role in human cancer.

The human carcinoembryonic antigen (CEA) GPI anchor mediates anoikis inhibition by inactivation of the intrinsic death pathway

Human carcinoembryonic antigen (CEA) is a cell surface adhesion molecule member of the Immunoglobulin Superfamily (IgSF). Aberrant upregulation of CEA is a common feature found in a wide variety of human cancers such as colon, breast and lung. Previous in vitro and in vivo results have demonstrated that CEA can have tumorigenic effects including the inhibition of cell differentiation and anoikis, a specific type of apoptosis triggered by the absence of extracellular matrix-cell contacts. In the present work, we investigate the involvement of the caspase cascade in CEA-mediated inhibition of anoikis and the structural requirements for this signal. Expression of CEA and/or a chimeric protein consisting of the NCAM extracellular domain attached to the CEA-GPI anchor correlates with an early inactivation of caspase-9 and activation of the PI3-K/Akt survival pathway, and at later times, inactivation of caspase-8. The CEA-mediated caspase inactivation as well as activation of Akt was not observed by expression of a CEA molecule incapable of self-binding (DeltaNCEA). These results suggest that the intrinsic caspase pathway is involved in the inhibitory effects of anoikis by CEA and this signal is dependent on the presence of self-adhesive extracellular domains and a CEA-GPI anchor.

A co-clustering model involving alpha5beta1 integrin for the biological effects of GPI-anchored human carcinoembryonic antigen (CEA)

CEA functions as an intercellular adhesion molecule and is up-regulated in a wide variety of human cancers, including colon, breast and lung. Its over-expression inhibits cellular differentiation, blocks cell polarization, distorts tissue architecture, and inhibits anoikis of many different cell types. Here we report results concerning the molecular mechanism involved in these biological effects, where relatively rapid molecular changes not requiring alterations in gene expression were emphasized. Confocal microscopy experiments showed that antibody-mediated clustering of a deletion mutant of CEA (DeltaNCEA), normally incapable of self binding and clustering, led to the co-localization of integrin alpha5beta1 with patches of DeltaNCEA on the cell surface. Activation of alpha5, as defined by an anti-alpha5 mAb-sensitive increase in cell adhesion to immobilized fibronectin, and an increased binding of soluble fibronectin to cells, was also observed. This was accompanied by the recruitment of integrin-linked kinase (ILK), protein kinase B (PKB/Akt), and the mitogen-activated protein kinase (MAPK) to membrane microdomains and the phosphorylation of Akt and MAPK. Inhibition of PI3-K and ILK, but not MAPK, prevented the alpha5beta1 integrin activation. Conversely, anti-alpha5 antibody inhibited the PI3-K-mediated activation of Akt, implying the involvement of outside-in and inside-out signaling in integrin activation. Therefore we propose that CEA-mediated signaling involves clustering of CEA and co-clustering and activation of the alpha5beta1 and associated specific signaling elements on the internal surfaces of membrane microdomains. These changes may represent a molecular mechanism for the biological effects of CEA.

Identification of a novel functional specificity signal within the GPI anchor signal sequence of carcinoembryonic antigen

Exchanging the glycophosphatidylinositol (GPI) anchor signal sequence of neural cell adhesion molecule (NCAM) for the signal sequence of carcinoembryonic antigen (CEA) generates a mature protein with NCAM external domains but CEA-like tumorigenic activity. We hypothesized that this resulted from the presence of a functional specificity signal within this sequence and generated CEA/NCAM chimeras to identify this signal. Replacing the residues (GLSAG) 6–10 amino acids downstream of the CEA anchor addition site with the corresponding NCAM residues resulted in GPI-anchored proteins lacking the CEA-like biological functions of integrin modulation and differentiation blockage. Transferring this region from CEA into NCAM in conjunction with the upstream proline (PGLSAG) was sufficient to specify the addition of the CEA anchor. Therefore, this study identifies a novel specificity signal consisting of six amino acids located within the GPI anchor attachment signal, which is necessary and sufficient to specify the addition of a particular functional GPI anchor and, thereby, the ultimate function of the mature protein.

Evolution of a tumorigenic property conferred by glycophosphatidyl-inositol membrane anchors of carcinoembryonic antigen gene family members during the primate radiation

GPI membrane anchors of cell surface glycoproteins have been shown to confer functional properties that are different from their transmembrane (TM)-anchored counterparts. For the human carcinoembryonic antigen (CEA) family, a subfamily of the immunoglobulin superfamily, conversion of the mode of membrane linkage from TM to GPI confers radical changes in function: from tumor suppression or neutrality toward inhibition of differentiation and anoikis and distortion of tissue architecture, thereby contributing to tumorigenesis. We show here that GPI anchorage in the CEA family evolved twice independently in primates, very likely from more primitive TM anchors, by different packages of mutations. Both mutational packages, one package found in many primates, including humans, and a second, novel package found only in the Cebidae radiation of New World monkeys, give rise to efficiently processed GPI-linked proteins. Both types of GPI anchors mediate inhibition of cell differentiation. The estimated rate of nonsynonymous mutations (Ka) in the anchor-determining domain for conversion from TM to GPI anchorage in the CEA family that were fixed during evolution in these primates is 7 times higher than the average Ka in primates, indicating positive selection. These results suggest therefore that the functional changes mediated by CEA GPI anchors, including the inhibition of differentiation and anoikis, could be adaptive and advantageous.

Specific inhibition of GPI-anchored protein function by homing and self-association of specific GPI anchors

The functional specificity conferred by glycophosphatidylinositol (GPI) anchors on certain membrane proteins may arise from their occupancy of specific membrane microdomains. We show that membrane proteins with noninteractive external domains attached to the same carcinoembryonic antigen (CEA) GPI anchor, but not to unrelated neural cell adhesion molecule GPI anchors, colocalize on the cell surface, confirming that the GPI anchor mediates association with specific membrane domains and providing a mechanism for specific signaling. This directed targeting was exploited by coexpressing an external domain-defective protein with a functional protein, both with the CEA GPI anchor. The result was a complete loss of signaling capabilities (through integrin–ECM interaction) and cellular effect (differentiation blockage) of the active protein, which involved an alteration of the size of the microdomains occupied by the active protein. This work clarifies how the GPI anchor can determine protein function, while offering a novel method for its modulation.

Increased colon tumor susceptibility in azoxymethane treated CEABAC transgenic mice

Human carcinoembryonic antigen (CEA), a widely used clinical tumor marker, and its close relative, CEACAM6, are often overexpressed in many cancers. This correlation suggests a possible instrumental role in tumorigenesis, which is supported by extensive results obtained with several in vitro systems. The implication that these results could also apply in vivo warrants investigation. Since mice do not possess homologs of the glycophosphatidyl inositol (GPI)-anchored CEACAM family genes CEA, CEACAM6 and CEACAM7, we have constructed transgenic mice harboring a 187 kb portion of the human CEACAM family gene locus contained in a bacterial artificial chromosome (CEABAC) that includes genes coding for CEA, CEACAM6 and CEACAM7. In this study, we treated the CEABAC mice and their wild-type littermates with azoxymethane (AOM) in order to induce colon tumor formation. At 20 weeks post-treatment, the CEABAC transgenics showed more than a 2-fold increase in mean tumor load relative to their wild-type littermates. Cell surface expression of CEA and CEACAM6 increased by 2- and 20-fold, respectively, in colonocytes from the tumors relative to colonocytes from non-AOM treated transgenics and a de-regulated spatial pattern of CEA/CEACAM6 expression was found in 'normal' crypts adjacent to the tumors, thus mimicking closely the situation in human colon tumorigenesis. A modestly increased incidence of beta-catenin mutations also observed in the AOM-induced CEABAC tumors. These results show that expression of the human GPI-anchored CEACAM family genes predisposes mice to acquire and/or retain essential mutations necessary for sporadic colon tumor development.

GPI-anchored CEA family glycoproteins CEA and CEACAM6 mediate their biological effects through enhanced integrin α5β1-fibronectin interaction

Carcinoembryonic antigen (CEA) and CEA family member CEACAM6 are glycophosphatidyl inositol (GPI)-anchored, intercellular adhesion molecules that are up-regulated in a wide variety of human cancers, including colon, breast, and lung. When over-expressed in a number of cellular systems, these molecules are capable of inhibiting cellular differentiation and anoikis, as well as disrupting cell polarization and tissue architecture, thus increasing tumorigenicity. The present study shows that perturbation of the major fibronectin receptor, integrin alpha5beta1, underlies some of these biological effects. Using confocal microscopy and specific antibodies, CEA and CEACAM6 were demonstrated to co-cluster with integrin alpha5beta1 on the cell surface. The presence of CEA and CEACAM6 was shown to lead to an increase in the binding of the integrin alpha5beta1 receptor to its ligand fibronectin, without changing its cell surface levels, resulting in increased adhesion of CEA/CEACAM6-expressing cells to fibronectin. More tenacious binding of free fibronectin to cells led to enhanced fibronectin matrix assembly and the formation of a polymerized fibronectin "cocoon" around the cells. Disruption of this process with specific monoclonal antibodies against either fibronectin or integrin alpha5beta1 led to the restoration of cellular differentiation and anoikis in CEA/CEACAM6 producing cells.

Novel mouse model for carcinoembryonic antigen-based therapy

Many novel cancer therapies, including immunotherapy and gene therapy, are specifically targeted to tumor-associated molecules, among which carcinoembryonic antigen (CEA) represents a popular example. Discrepancies between preclinical experimental data in animal models and clinical outcome in terms of therapeutic response and toxicity, however, often arise. Preclinical testing can be compromised by the lack of CEA and other closely related human CEA family members in rodents, which lack analogous genes for most human CEA family members. Here, we report the construction of a transgenic mouse with a 187-kb human bacterial artificial chromosome (CEABAC) that contains part of the human CEA family gene cluster including complete human CEA (CEACAM5), CEACAM3, CEACAM6, and CEACAM7 genes. The spatiotemporal expression pattern of these genes in the CEABAC mice was found to be remarkably similar to that of humans. This novel mouse will ensure better assessment than previously utilized models for the preclinical testing of CEA-targeted therapies and perhaps allow the testing of CEACAM6, which is overexpressed in many solid tumors and leukemias, as a therapeutic target. Moreover, expression of CEA family genes in gastrointestinal, breast, hematopoietic, urogenital, and respiratory systems could facilitate other clinical applications, such as the development of therapeutic agents against Neisseria gonorrhoeae infections, which use CEA family members as major receptors.

Minimal mutations are required to effect a radical change in function in CEA family members of the Ig superfamily

GPI anchorage in the CEA family results in the acquisition of radically different functions relative to TM anchorage, including inhibition of differentiation and anoikis, disruption of tissue architecture and promotion of tumorigenicity. CEA GPI anchors, as determined by the carboxy-terminal exon of CEA, demonstrate biological specificity in their ability to confer these functional changes. CEA family GPI anchorage appears to have evolved twice independently during the primate radiation, in a manner suggestive of evolution from more primitive TM-anchored CEACAM1. We show here that very few mutations in the TM exon of present-day human CEACAM1 are required to give efficient GPI anchorage and the biological specificity of CEA GPI anchors, i.e., to give the differentiation-blocking function of GPI-anchored CEA. Such a change in anchorage could therefore represent a relatively facile means for producing radical change in molecular function of Ig superfamily members during evolution.

The adhesion and differentiation-inhibitory activities of the immunoglobulin superfamily member, carcinoembryonic antigen, can be independently blocked

The external domains of Ig superfamily members are involved in multiple binding interactions, both homophilic and heterophilic, that initiate molecular events leading to the execution of diverse cell functions. Human carcinoembryonic antigen (CEA), an Ig superfamily cell surface glycoprotein used widely as a clinical tumor marker, undergoes homophilic interactions that mediate intercellular adhesion. Recent evidence supports the view that deregulated overexpression of CEA has an instrumental role in tumorigenesis through the inhibition of cell differentiation and the disruption of tissue architecture. The CEA-mediated block of the myogenic differentiation of rat L6 myoblasts depends on homophilic binding of its external domains. We show here that L6 transfectant cells expressing CEA can "trans-block" the myogenesis of juxtaposed differentiation-competent L6 transfectant cells expressing a deletion mutant of CEA (DeltaNCEA). This result implies the efficacy of antiparallel CEA-CEA interactions between cells in the differentiation block. In addition, DeltaNCEA can acquire differentiation blocking activity by cross-linking with specific anti-CEA antibodies, thus implying the efficacy of parallel CEA-CEA interactions on the same cell surface. The myogenic differentiation blocking activity of CEA was demonstrated by site-directed mutations to involve three subdomains of the amino-terminal domain, shown previously to be critical for its intercellular adhesion function. Monovalent Fab fragments of monoclonal antibodies binding to the region bridging subdomains 1 and 2 could both inhibit intercellular adhesion and release the myogenic differentiation block. Amino acid substitutions Q80A, Q80R, and D82N in subdomain 3, QNDTG, however, were found to completely ablate the differentiation blocking activity of CEA but had no effect on intercellular adhesion activity. A cyclized peptide representing this subdomain was the most effective at releasing the differentiation block.

Deregulated expression of the human tumor marker CEA and CEA family member CEACAM6 disrupts tissue architecture and blocks colonocyte differentiation

Human carcinoembryonic antigen (CEA) and the CEA family member CEACAM6 (formerly nonspecific cross-reacting antigen [NCA]) function in vitro, at least, as homotypic intercellular adhesion molecules and, in model systems, can block the terminal differentiation and anoikis of several different cell types. We have recently demonstrated that the increased cell surface levels of CEA and CEACAM6 in purified human colonocytes from freshly excised, well to poorly differentiated colon carcinomas are inversely correlated with the degree of cellular differentiation. Thus, deregulated expression of CEA/CEACAM6 could directly contribute to colon tumorigenesis by the inhibition of terminal differentiation and anoikis. Evidence against this view includes the common observation of increased CEA/CEACAM6 expression as normal colonocytes differentiate in their migration up colonic crypt walls. We report here the direct effects of deregulated overexpression of CEA/CEACAM6, at levels observed in colorectal carcinomas, on the differentiation of two human colonic cell lines, SW-1222 and Caco-2. Stable transfectants of both of these cell lines that constitutively express 10- to 30-fold higher cell surface levels of CEA/CEACAM6 than endogenous levels failed to polarize and differentiate into glandular structures in monolayer or 3D culture or to form colonic crypts in a tissue architecture assay in nude mice. In addition, these transfectants were found to exhibit increased tumorigenicity in nude mice. These results thus support the contention that deregulated overexpression of CEA and CEACAM6 could provide a tumorigenic contribution to colon carcinogenesis.

Different host-cell shutoff strategies related to the matrix protein lead to persistence of vesicular stomatitis virus mutants on fibroblast cells

Acute infection of fibroblastic cell lines by the Indiana strain of vesicular stomatitis virus (VSV) usually induces dramatic cytopathic effects and shutoff of cellular gene expression. We have compared a series of independent mutants with differences in shutoff induction and found that M was mutated either in the N-terminus (M(51)R) or C-terminus (V(221)F and S(226)R). Furthermore, only double mutants (M mutation and a ts mutation related or not to M) were able to persist on fibroblast cell lines at 39 degrees C. A more detailed investigation of the infection was performed for the mutants T1026, TP3 and G31, differing in their host shutoff effects related to M protein. Viral activity in persistently infected mouse L-929 and monkey Vero cell lines was followed by viral proteins detection, RNA synthesis throughout infection and finally detection of infectious particles. All three mutants cause extensive CPE followed by emergence of persistently infected cells on Vero cells. The same thing is seen on L-929 cells except for T1026 which causes little CPE. Taken together, the results form a basis of further studies to clarify how various viral and cellular factors interact in the establishment of a persistent infection by VSV mutants.

Soluble isoforms of CEACAM1 containing the A2 domain: increased serum levels in patients with obstructive jaundice and differences in 3-fucosyl-N-acetyl-lactosamine moiety

CEACAM1 (biliary glycoprotein or CD66a) is a member of the carcinoembryonic antigen (CEA) subgroup of the CEA family. Eleven RNA isoforms derived from the splicing of a single CEACAM1 gene have been described. Some of the CEACAM1 isoforms have been recognized by the CD66 antibodies in T and B lymphocytes, natural killer cells, granulocytes and epithelial cells in several human tissues. Although it is also present in soluble form in bile and serum, and elevated levels have been found in the serum of patients with liver diseases, it is not known which isoforms are primarily involved. In order to learn more about the distribution and properties of particular CEACAM1 isoforms, we have prepared a monoclonal antibody specific for the A2 domain of CEACAM1, designated TEC-11. This antibody does not cross-react with other members of the CEA family. Immunoblotting analysis revealed that the TEC-11 epitope was present in all cell types expressing CEACAM1 containing the A2 domain [CEACAM1(A2)], including granulocytes (160 000 MW isoform) and sperm cells (140 000 MW isoform). A 115 000 MW isoform of CEACAM1(A2) was present in human serum, bile, saliva and seminal fluid. Human bile, saliva and seminal fluid also contained the 160 000 MW CEACAM1(A2) isoform. Significantly higher serum levels of the 115 000 MW CEACAM1(A2) isoform were detected in patients with obstructive jaundice. The 160 000 MW isoform of CEACAM1(A2) in bile, but not a 115 000 MW isoform in serum and bile, carried the 3-fucosyl-N-acetyl-lactosamine moiety. The combined data indicate that various isoforms of CEACAM1(A2) are present in different body fluids where they could take part in different CEACAM1-mediated functions.

Self recognition in the Ig superfamily. Identification of precise subdomains in carcinoembryonic antigen required for intercellular adhesion

The homophilic binding of extracellular domains of membrane-bound immunoglobulin superfamily (IgSF) molecules is often required for intercellular adhesion and signaling. Carcinoembryonic antigen (CEA), a member of the IgSF, is a widely used tumor marker that functions in vitro as a homotypic intercellular adhesion molecule. CEA has also been shown to contribute to tumorigenicity by inhibiting cellular differentiation, an effect that requires the homophilic binding of its extracellular domains. It was of interest, therefore, to identify small subdomain sequences in CEA that could serve as a focus in the design of peptides that disrupt CEA-mediated intercellular adhesion. Three subdomains in the N-terminal domain of CEA, identified by site-directed deletions and point mutations, were shown to be required for intercellular adhesion. Cyclized peptides representing two of these subdomains, (42)NRQII and (80)QNDTG, were found to be effective in blocking CEA-mediated cellular aggregation when added to CEA-expressing transfectants in suspension. Intermolecular binding involving each of these subdomains is therefore essential for intercellular adhesion and cannot be compensated for by known binding contributions of other regions in the CEA molecule. In further support of this assumption, the binding epitope of an anti-CEA monoclonal antibody (monoclonal antibody A20) known to block CEA-mediated adhesion, was shown to bridge two of the three required subdomains: (42)NRQII and (30)GYSWYK.

The specificity for the differentiation blocking activity of carcinoembryonic antigen resides in its glycophosphatidyl-inositol anchor

Ectopic expression of various members of the human carcinoembryonic antigen (CEA) family of intercellular adhesion molecules in murine myoblasts either blocks (CEA, CEACAM6) or allows (CEACAM1) myogenic differentiation. These surface glycoproteins form a subset of the immunoglobulin (Ig) superfamily and are very closely related, but differ in the precise sequence of their external domains and in their mode of anchorage to the cell membrane. CEA and CEACAM6 are glycophosphatidyl-inositol (GPI) anchored, whereas CEACAM1 is transmembrane (TM) anchored. Overexpression of GPI-linked neural cell adhesion molecule (NCAM) p125, also an adhesion molecule of the Ig superfamily, accelerates myogenic differentiation. The molecular requirements for the myogenic differentiation block were investigated using chimeric constructs in which the COOH-terminal hydrophobic domains of CEA, CEACAM1, and NCAM p125 were exchanged. The presence of the GPI signal sequence specifically from CEA in the chimeras was sufficient to convert both CEACAM1 and NCAM into differentiation-blocking proteins. Conversely, CEA could be converted into a neutral protein by exchanging its GPI anchor for the TM anchor of CEACAM1. Since the external domains of CEA, CEACAM1, and NCAM can all undergo homophilic interactions, and mutations in the self-adhesive domains of CEA abrogate its differentiation-blocking activity, the structural requirements for differentiation-inhibition are any self-adhesive domains attached to the specific GPI anchor derived from CEA. We therefore suggest that biologically significant functional information resides in the processed extreme COOH terminus of CEA and in the GPI anchor that it determines.

T-cell killing of heterogenous tumor or viral targets with bispecific chimeric immune receptors

We have previously described several novel chimeric immune receptors (CIRs) that redirect human T cells to kill malignant or HIV-infected cells. These CIRs comprise a cancer- or virus-specific ligand or single-chain antibody fused to the signaling domain of the T-cell receptor CD3-zeta subunit. Binding of the ligand- or antibody-based CIR to the target antigen (Ag) triggers T-cell-mediated cytolysis of the tumor- or virus-infected cell independent of target cell major histocompatibility complex class I expression. A new type of CIR was developed to mediate the lysis of cells that expressed one or more distinct viral or tumor Ags; three bispecific CIRs (BCIRs) were generated that recognized the carcinoembryonic Ag (CEA) and TAG-72 tumor Ags or, alternatively, distinct epitopes in the HIV envelope (HIVenv). T cells expressing the antitumoral Ag BCIR lysed both CEA- and TAG-72-expressing targets and did not kill Ag-negative targets or target cells expressing other members of the CEA family. Similarly, T cells expressing the anti-HIVenv BCIR lysed target cells expressing both the wild-type HIVenv and a mutant HIVenv that lacked the epitopes recognized by the monospecific CIRs. This approach permits the generation of T cells with a broader spectrum of activity capable of killing virus-infected cells and malignant cells and reduces the potential of progression of disease due to Ag loss variants.

Human carcinoembryonic antigen functions as a general inhibitor of anoikis

Human carcinoembryonic antigen (CEA), a widely used tumor marker, and CEACAM6 [formerly nonspecific cross-reacting antigen (NCA)] are up-regulated in many types of human cancers, whereas family member CEACAM1 [formerly biliary glycoprotein (BGP)] is usually down-regulated. Deregulated overexpression of CEA/CEACAM6 but not CEACAM1 can inhibit the differentiation and disrupt the polarization and tissue architecture of many different types of cells. In this report, we show that CEA and CEACAM6, but not CEACAM1, markedly inhibit the apoptosis of cells when deprived of their anchorage to the extracellular matrix, a process known as anoikis. By blocking this tissue architecture surveillance mechanism, the architectural perturbation initiated by CEA/CEACAM6 can thus be maintained.

Role of carbohydrate structures in CEA-mediated intercellular adhesion

Human carcinoembryonic antigen (CEA) is a member of a family of cell surface glycoproteins representing a subset of the immunoglobulin superfamily and is a major tumor marker. CEA has been demonstrated to function in vitro, at least, as a homotypic intercellular adhesion molecule. CEA can also inhibit the differentiation of several different cell types and contribute to tumorigenesis, an activity that requires CEA-CEA interactions. Post-translational modifications that could modulate CEA-CEA binding are therefore of interest. CEA is heavily glycosylated with 28 consensus sites for the addition of asparagine-linked carbohydrate structures, leading to a molecule with a bottle brush-like structure. In order to modulate the glycosylation of CEA, we transfected the functional cDNA of CEA into Chinese hamster ovary (CHO) mutant cells, Lec1, Lec2, and Lec8, which are deficient in enzymes responsible for various steps in the glycosylation processing pathway. Aggregation assays of cells in suspension were performed with stable CEA transfectants of these cell lines and showed that all of the aberrant CEA glycoforms could still mediate adhesion. In addition, the specificity of adhesion of these glycoforms was unchanged, as shown by homotypic and heterotypic adhesion assays between the transfectants. Lec1 and Lec2 transfectants did, however, show an increased speed and final extent of aggregation, which is consistent with models in which sugar structures interfere with binding through protein domains. Lec8 transfectants, on the other hand, with more truncated sugar structures than Lec2, showed less aggregation than wild type (WT) transfectants. We therefore conclude that carbohydrates do not determine the adhesion property of CEA or its specificity, in spite of the unusually high degree of glycosylation; they do, however, modulate the strength of adhesion.

Transcriptional regulation of the non-specific cross-reacting antigen gene, a member of the carcinoembryonic antigen gene family up-regulated in colorectal carcinomas

Human non-specific cross-reacting antigen (NCA), a close relative of the tumor marker human carcinoembryonic antigen (CEA), is also an in vitro homotypic intercellular adhesion molecule capable of inhibiting differentiation when expressed ectopically by myoblasts. Moreover, NCA appears to be overexpressed at the transcriptional level to a greater extent and more frequently in colorectal carcinomas than CEA. This study examines the transcriptional control mechanisms responsible for orchestrating NCA expression. The region within 284 bp upstream of the translational start site of the NCA gene was found to be capable of directing high levels of expression in functional promoter assays. Footprinting experiments identified three cis-acting elements and mobility-shift assays revealed that the first of these elements is bound by the upstream stimulating factors USF1 and USF2 while the other two are bound by the stimulatory proteins Sp1 and Sp3. No cis-acting elements corresponding to CEA footprint FP4 or the silencer CEA FP5 were detected in the NCA promoter, which may contribute to the differential expression of NCA versus CEA in tumorigenesis.

Carcinoembryonic antigen, a human tumor marker, cooperates with Myc and Bcl-2 in cellular transformation

Carcinoembryonic antigen (CEA) is a tumor marker that is overexpressed in many human cancers and functions in vitro as a homotypic intercellular adhesion molecule. We have investigated the possibility of synergy between CEA, v-Myc, and Bcl-2 in the transformation of cells with differentiation capacity. We find that v-Myc increases the cell division rate and maximum density of rat L6 myoblasts but also markedly stimulates both apoptosis and surprisingly, differentiation, thus preventing transformation. The superposition of Bcl-2 blocks the apoptotic stimulation of v-Myc and independently promotes further cell division at confluence, but still allows differentiation. The further expression of CEA has a dominant effect in blocking differentiation, regardless of the presence of the other activated oncogenes, generating cells that enter a reversible quiescent G0-like state in medium promoting differentiation. Transfectants expressing CEA with or without v-myc and bcl-2 allow the emergence of cells with the property of heritable, efficient, anchorage-independent growth in soft agar and the ability to markedly reduce the latency for tumor formation in nude mice. We propose that by prolonging cell survival in the presence of differentiation signals, CEA represents a novel class of dominant differentiation-blocking oncogene.

Cell-surface levels of human carcinoembryonic antigen are inversely correlated with colonocyte differentiation in colon carcinogenesis

Human carcinoembryonic antigen (CEA) is overexpressed in a wide variety of epithelial malignancies including colon cancer. CEA can function in vitro as a homotypic intercellular adhesion molecule and can block the terminal differentiation of rodent myoblasts, thus raising the possibility that deregulated expression of CEA might directly contribute to malignant progression. To address this question, the expression pattern and cell-surface levels of CEA were studied during malignant transformation of the colonic epithelium in sporadic and familial adenomatous polyposis-related neoplasms. The level of immunohistochemically detected CEA was higher in 30% to 62% of microadenomas and small adenomas from familial adenomatous polyposis patients compared with adjacent normal mucosa, and this proportion was positively correlated with lesion size and degree of dysplasia. Cytofluorometric analysis of highly purified single epithelial cell suspensions from freshly excised carcinomas versus adjacent normal tissue demonstrated up to a 20-fold increase of mean cell-surface CEA in a group of colon carcinomas representative of the overall majority of such tumors--from Dukes' stages A to D and ranging mainly from well to moderately differentiated, the degree of overproduction was inversely correlated with tumor differentiation and directly correlated with stage. A marked tendency toward nonpolarized versus apical cell-surface expression with progression was also noted. Nonspecific cross-reacting antigen (NCA), a CEA family member, is also a homotypic adhesion molecule and blocks terminal myogenic differentiation, whereas biliary glycoprotein is a CEA family adhesion molecule that does not. Cell-surface NCA showed even greater overexpression (up to 70-told) in dedifferentiated tumors, whereas total-cell biliary glycoprotein showed approximately 2-fold lower levels than was normal in more differentiated tumors and approximately 2-fold higher levels than in further progressed tumors. These results therefore support the suggestion that CEA and NCA can directly contribute to colon carcinogenesis by inhibiting colonocyte differentiation.

A colonic tissue architecture assay applied to human colon carcinoma cells

A two-component tissue architecture assay system has been devised that tests the ability of human colon carcinoma cells to conform to the specific three-dimensional cell-cell and cell-substratum interactions characteristic of normal colonic tissues. Dissociated fetal rat colonic cells (FRCC) were allowed to reaggregate in suspension with or without the addition of different proportions (0.1%, 1%, and 10% of the total cells) of the human colon carcinoma cell lines, SW-1222 and LS-174T. Cellular aggregates obtained after 36 hours, incubation exhibited cell sorting by the formation of recognizable epithelial colonic crypt-like structures with glandular lumens in a mesenchyme-like background. Carcinoembryonic antigen (CEA)-positive SW-1222 cells in 10% mixed aggregates were organized into numerous well-formed glandular structures with a polarized apical distribution of CEA. LS-174T cells, on the other hand, were self-sorted but structurally disorganized with a continuous cell surface CEA distribution. Pure FRCC and mixed aggregates were implanted under the kidney capsules of Swiss nu/nu (nude) or CD-1 nu/nu mice and allowed to grow for a period of 7-10 days. Whereas the normal FRCC readily formed colonic tissue, the SW-1222 cells exhibited a capacity for differentiation into colonic crypts which became progressively less normal and more tumor-like as the proportion of carcinoma cells in the aggregates was increased. The LS-174T cells demonstrated poor differentiation at all concentrations. Cell surface levels of CEA and the CEA family member nonspecific crossreacting antigen (NCA), both overexpressed in colon cancer, were higher in LS-174T than in SW-1222 cells, whereas family member biliary glycoprotein (BGP), downregulated in colon carcinoma was higher in the SW-1222 cells. These results thus support the suggestion that deregulated expression of CEA family members can be involved in the ability of colonocytes to differentiate and conform to normal tissue architecture as assessed by the assay. The assay is therefore amenable to genetic analysis of normal and perturbed architectural phenotypes.

Radical differences in functions of closely related members of the human carcinoembryonic antigen gene family

The immunoglobulin superfamily represents an ancient, highly diversified group of cell surface and extracellular molecules responsible for a wide range of molecular and cellular recognition functions. The human carcinoembryonic antigen (CEA) subfamily of the immunoglobulin superfamily presents evidence of continuing diversification of the immunoglobulin family, in that some of its members, including CEA itself and nonspecific cross-reacting antigen (NCA), are expressed only in primates and not in rodents. These "new" members are glycophosphatidylinositol linked to the external cell membrane and are up-regulated in cancer, unlike members present in both rodents and primates, i.e., biliary glycoprotein (BGP), which are transmembrane linked and down-regulated in cancer. CEA, NCA, and BGP have all been shown to function in vitro as intercellular adhesion molecules. We show here that the properties of adhesion are radically different, in that BGP-mediated adhesion is reversibly Ca2+ and Mg2+ dependent, temperature dependent, and ATP inhibitable, whereas CEA- and NCA-mediated adhesion is the opposite in all aspects. Also, the novel double-reciprocal, antiparallel binding observed for CEA-CEA interactions is not seen for BGP. Finally, the myogenic differentiation block demonstrated for the ectopic expression of CEA in myoblasts was also observed for NCA but not for BGP, which is consistent with the changes in expression seen in cancer. The appearance of new CEA family members with such different properties is discussed in the context of evolution and cancer.

Transcriptional regulation of the carcinoembryonic antigen gene. Identification of regulatory elements and multiple nuclear factors

Human carcinoembryonic antigen (CEA) belongs to a family of membrane glycoproteins that are overexpressed in many carcinomas; CEA functions in vitro as a homotypic intercellular adhesion molecule and can inhibit differentiation when expressed ectopically in myoblasts. The regulation of expression of CEA is therefore of considerable interest. The CEA gene promoter region between -403 and -124 base pairs upstream of the translation initiation site directed high levels of expression in CEA-expression SW403 cells and was 3 times more active in differentiated than in undifferentiated Caco-2 cells, correlating exactly with the 3-fold increase in CEA mRNA seen in differentiated Caco-2 cells. Inclusion of additional upstream sequences between -1098 and -403 base pairs repressed all activity. By in vitro footprinting and deletion analyses, four cis-acting elements were mapped within the positive regulatory region, and one element within the silencing region. Several nuclear factors binding to these domains were identified: USF, Sp1, and an Sp1-like factor. By co-transfection, USF directly activated the CEA gene promoter in vivo in both SW403 and Caco-2 cells. In addition, the levels of factors binding to each positively acting element increased dramatically with differentiation in Caco-2 cells. Thus the transcriptional control of the CEA gene depends on the interaction of several regulatory elements that bind multiple specific factors.

Opposite functions for two classes of genes of the human carcinoembryonic antigen family

The human carcinoembryonic antigen (CEA) family can be divided into two subgroups according to the means of anchorage of member glycoproteins to the cell membrane: glycophosphatidyl inositol (GPI) linkage and transmembrane linkage. The GPI-linked members tend to be up-regulated in human tumours, whereas the transmembrane-linked members tend to be down-regulated. Thus the question as to whether the GPI members could be formally considered to function as oncogenes and the transmembrane members as tumour suppressors deserves consideration. Members of both subgroups function in vitro as intercellular adhesion molecules, but the characteristics of this adhesion, including temperature and divalent-cation dependence, differ markedly between the groups. Even the mechanism of intermolecular adhesion appears to differ fundamentally in that GPI-linked CEA-CEA binding involves a double reciprocal bonding between two domains, whereas transmembrane-linked biliary glycoprotein (BGP)-BGP binding requires only one domain. Finally, the ectopic expression of CEA in myoblasts can block myogenic differentiation leaving the cells with the ability to divide, while expression of BGP does not affect or may even accelerate myogenic differentiation. These differences in phenotypic effects in vitro thus mirror the differences observed in expression in tumours and support the view that the GPI and transmembrane groups have opposite effects on cells in relation to the malignant phenotype.

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.

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.

Adhesion to carcinoembryonic antigen by human colorectal carcinoma cells involves at least two epitopes

Carcinoembryonic antigen (CEA) may be involved in both cell-cell and cell-substrate adhesion. Our purpose was to determine whether epitopes involved in the homophilic binding of human colorectal carcinoma cells to CEA participated in adhesion to basement membrane proteins. Three human colorectal adenocarcinoma cell lines and one CHO cell line transfected with CEA cDNA were tested in a solid-phase adhesion assay. The 2 CEA-expressing carcinoma cell lines (KM-12c and CCL 188) and the transfectant, but not the parental CHO line, bound to CEA. The CEA-non-producing carcinoma line (Clone A) did not bind to CEA. All colorectal carcinoma cell lines, the transfectant and the parental CHO line bound to laminin, while the colorectal carcinoma lines bound to type-IV collagen. MAbs to epitopes on CEA that cross-react with non-specific cross-reacting antigen (NCA) inhibited adhesion of CEA-expressing cells to CEA. MAbs to non-cross-reactive epitopes of CEA did not block adhesion to CEA. When the inhibitory anti-CEA antibodies were compared in a competitive radioimmunoassay, 2 distinct epitopes were identified. Epitope I is in the N-terminal domain and defined by MAbs MN3, T84.1 and C110, whereas epitope II is located in the repeating loop domains and is recognized by antibodies MN15, PR3B10 and NP1. None of the antibodies to epitope I or II blocked adhesion by KM-12c or CCL 188 cells to laminin or type-IV collagen. Thus, at least 2 different regions on CEA participate in adhesion to CEA but not to collagen or laminin by CEA-expressing human colorectal carcinoma cells.

Specificity of anti-carcinoembryonic antigen monoclonal antibodies and their effects on CEA-mediated adhesion

The carcinoembryonic antigen (CEA) domain specificities of a library of monoclonal anti-CEA antibodies were determined to investigate the mechanisms of homophilic binding involved in CEA-mediated intercellular adhesion. Using an indirect immunofluorescence cell surface staining technique, the reactivities of these antibodies were tested systematically on Chinese hamster ovary cells stably transfected with constructs of CEA gene family members CEA, nonspecific cross-reacting antigen, CGM6, and biliary glycoprotein, as well as on stable Chinese hamster ovary transfectants expressing truncated CEA and CEA/nerve cell adhesion molecule chimeric proteins. Epitopes for these antibodies were thus localized on the CEA molecule as follows: monoclonal antibody groups 1, 2, and 6 react with epitopes in the N-terminal domain of CEA, whereas groups 3, 5a, 5b, and 9 react with determinants found in the internal repeating domains located in the C-terminal part of the CEA molecule. Groups 4 and 8 appear to react with a repeating epitope in the internal domains of CEA and in all CEA subfamily members. Fab fragments of monoclonal antibody group 1 block aggregation of CEA transfectant cells, indicating that a region in the N domain is involved in CEA homophilic interaction. Fab fragments of monoclonal antibody group 9 recognize an epitope dependent on the B2 and A3 domains and stimulate aggregation. These results support the recently reported CEA-CEA homophilic reciprocal binding model in which anti-parallel molecules are bound by 2 bonds between the N domain of one molecule and the A3B3 domain of the interacting partner.

Homophilic adhesion between Ig superfamily carcinoembryonic antigen molecules involves double reciprocal bonds

Both carcinoembryonic antigen (CEA) and neural cell adhesion molecule (NCAM) belong to the immunoglobulin supergene family and have been demonstrated to function as homotypic Ca(++)-independent intercellular adhesion molecules. CEA and NCAM cannot associate heterotypically indicating that they have different binding specificities. To define the domains of CEA involved in homotypic interaction, hybrid cDNAs consisting of various domains from CEA and NCAM were constructed and were transfected into a CHO-derived cell line; stable transfectant clones showing cell surface expression of CEA/NCAM chimeric-proteins were assessed for their adhesive properties by homotypic and heterotypic aggregation assays. The results indicate that all five of the Ig(C)-like domains of NCAM are required for intercellular adhesion while the COOH-terminal domain containing the fibronectin-like repeats is dispensable. The results also show that adhesion mediated by CEA involves binding between the Ig(V)-like amino-terminal domain and one of the Ig(C)-like internal repeat domains: thus while transfectants expressing constructs containing either the N domain or the internal domains alone were incapable of homotypic adhesion, they formed heterotypic aggregates when mixed. Furthermore, peptides consisting of both the N domain and the third internal repeat domain of CEA blocked CEA-mediated cell aggregation, thus providing direct evidence for the involvement of the two domains in adhesion. We therefore propose a novel model for interactions between immunoglobulin supergene family members in which especially strong binding is effected by double reciprocal interactions between the V-like domains and C-like domains of antiparallel CEA molecules on apposing cell surfaces.

Identification of a human cancer related organ-specific neoantigen

Human cancers express organ-specific neoantigens (OSNs) which elicit specific cellular immune responses in the cancer patient, as demonstrated by leukocyte adherence inhibition (LAI), an in vitro immune response assay. A purified protein of MW 40,000 (p40) exhibiting OSN (colon specific) activity was cleaved into specific peptide fragments and their partial amino acid sequences determined. This information was used in the polymerase chain reaction (PCR) to obtain a 992 bp cDNA clone (PCR-992) from a human colon adenocarcinoma cell line (LS-180). By comparison of the predicted amino acid sequence of PCR-992 with the known sequence of p40 peptides, PCR-992 was shown to correspond to almost the entire coding region of p40. Nucleotide sequence analysis suggested that the protein was mycoplasmal in origin due to its high A+T content (76%) and the presence of five in frame TGA termination codons; at least two of the latter are actually read as tryptophan, a known feature of mycoplasma translation. We have confirmed this origin by direct isolation of a contaminating mycoplasma species from the LS-180 cell line and demonstration that it could be hybridized with the PCR-992 probe. Northern and PCR analysis of RNA preparations from the contaminated LS-180 cell line showed that p40 was part of the high affinity transport system operon of Mycoplasma hyorhinis (Dudler et al, EMBO J., 7: 3963-3970, 1988). Total protein lysates of Mycoplasma hyorhinis cultivated without animal cells could elicit positive LAI responses when incubated with cancer patient leukocytes but not with normal patient leukocytes. The organ-specific nature of the response was, however, not observed indicating that host cell-mycoplasmal interactions may play a role in determining the organ-specific nature of p40 seen with the LAI. The significance of these findings will be discussed in the context of previous thinking regarding the origin of OSNs.

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

Biliary-glycoprotein (BGP), a cell adhesion molecule related to carcinoembryonic antigen (CEA), has been shown to exist as several alternatively spliced isoforms. Here we show that BGPa and BGPb are phosphorylated in the chronic myelogenous leukaemia cell line KG-1, which constitutively expresses several BGP isoforms, and Chinese hamster LR-73 cells transfected with the cDNAs encoding BGPa and BGPb. The phosphorylation can be augmented with the protein tyrosine phosphatase inhibitor ammonium vanadate and with TPA (an activator of protein kinase C). Phospho-amino acid analysis of phosphorylated BGPs demonstrated that phosphorylation occurs on serine, threonine and tyrosine residues. Phosphorylation reactions carried out in in vitro membrane preparations from KG-1 cells revealed a close association of BGP proteins with membrane associated protein tyrosine kinases. These observations suggest an association of BGP proteins with signal transduction molecules which is regulated by alternative splicing of the cytoplasmic domain.

Cell adhesion properties of myelin-associated glycoprotein in L cell fibroblasts

Myelin-associated glycoprotein (MAG) is a cell surface molecule expressed by oligodendrocytes and Schwann cells. In order to determine whether MAG expression can confer adhesive properties to cells which normally do not aggregate in suspension, the cDNA encoding the long form of MAG (L-MAG) was introduced into L cell fibroblasts by retroviral infection. Clonal L cell lines expressing MAG were then subjected to a cell aggregation assay. Our results indicate that L-MAG can function as an intercellular adhesion molecule in a heterologous cell system. A critical threshold value of L-MAG expression was required for cell aggregation to occur. The adhesive properties of these cells were specific to MAG, since monoclonal antibodies directed against its extracellular domain inhibited aggregation. Furthermore, the adhesion was found to be calcium- and temperature-independent. Cell sorting experiments demonstrated that L-MAG-expressing cells bind in a heterotypic fashion to parental L cell fibroblasts. These results suggest that L-MAG can function as a heterotypic cell adhesion molecule recognizing a cell surface molecule(s) expressed by L cells.

Control of carcinoembryonic antigen gene family expression in a differentiating colon carcinoma cell line, Caco-2

The levels of control involved in the regulation of expression of the carcinoembryonic antigen (CEA) gene family members were investigated using cultured Caco-2 cells that differentiated after confluence as characterized by the production of polarized monolayers and the subsequent appearance of domes. Three transcripts representing CEA (3.0 and 3.5 kilobases) and nonspecific cross-reacting antigen (NCA) (2.6 kilobases) were detected in Northern analyses of mRNA preparations of such cells when probed with human CEA cDNA, albeit at different levels. The major CEA 3.0-kilobase transcript increased 3-fold over an 11-day culture period after confluence, whereas the NCA transcript increased 25-fold over the same time period. gamma-interferon treatment enhanced CEA mRNA levels 32-fold and NCA mRNA levels 53-fold in Caco-2 monolayers 11 days after confluence. The NCA gene thus appears to be regulated by a mechanism different from that of CEA. During gamma-interferon treatment, the normal increase in Caco-2 dome formation with time in culture was increased further by a factor of 2. Over the 13-day time span for Caco-2 cultures, CEA protein levels increased 7-fold, NCA (Mr 48,000) protein 5-fold, while gamma-interferon treatment augmented CEA 18.5-fold further and NCA 20-fold. In 2 other colon carcinoma cell lines, SW1222 and T84, which are differentiated in culture to varying degrees, little if any changes were seen in CEA and NCA mRNA and protein levels in pre- versus postconfluent cultures. Both cell lines, however, responded to gamma-interferon treatment by increases in CEA and NCA mRNA levels and, in some cases, disproportionate increases in the corresponding proteins. The lack of direct proportionality between mRNA and protein expression suggests that, as observed in human colon carcinoma and adjacent normal tissue, both transcriptional and post-transcriptional control mechanisms regulate CEA gene family member expression in colon carcinoma cells.

Sequence requirements for the stimulation of gene amplification by a mammalian genomic element

HSAG-1 is a 3.4-kb genomic element from a human chronic lymphocytic leukemia--Chinese hamster ovary (CHO) hybrid cell line shown to stimulate the amplification of expression vectors in cis when transfected into a variety of cell lines [McArthur and Stanners, J. Biol. Chem. 266 (1991) 6000-6005]. Subfragments of HSAG-1 were tested for amplification activity by insertion into the vector, pSV2DHFR. The results suggest that multiple positive- and negative-acting elements were present that influenced amplification activity. The deletion of regions believed to contain positive-acting elements decreased or abolished the amplification stimulatory activity of the most active 1.45-kb fragment, supporting this hypothesis. The construction of composite sequences containing multiple positive elements and lacking negative elements, however, failed to enhance the activity; maximum activity was obtained only with the original intact configuration of elements. Two of two CHO HSAG-1-like elements tested had an activity equivalent to HSAG-1, while one of 24 random CHO genomic fragments tested had an activity as high as HSAG-1. The combination of sequence and structural features needed to affect the frequency of gene amplification may therefore be quite common in the mammalian genome.

Elements which stimulate gene amplification in mammalian cells: role of recombinogenic sequences/structures and transcriptional activation

HSAG-1 is a 3.4 kb mammalian genomic element which has been shown to stimulate the amplification of the pSV2DHFR expression vector in cis when transfected into a variety of cell lines (1). This amplification stimulatory activity requires the interaction of multiple positive acting elements that include sequence features associated with recombination 'hotspots', such as Alu-like repetitive sequences and A/T rich regions (2). We demonstrate here that two other members of the HSAG family of elements, HSAG-2 and HSAG-5, also stimulate vector amplification. By analysis of the HSAG-2 nucleotide sequence and of the amplification activity of HSAG-2 and HSAG-5 subfragments, we show that this activity also involves the interaction of multiple positive acting elements. The autonomous replication of the HSAG containing vectors is not responsible for this effect. We also show that the orientation of HSAG elements in pSV2DHFR has a profound effect on their amplification stimulatory activity, and present evidence that the transcription of these elements in pSV2DHFR is necessary for the effect.

A genetic element that increases the frequency of gene amplification

A repetitive mammalian genetic element, HSAG-1, has been shown to promote the amplification of the vector, pSV2-DHFR, containing the functional cDNA for dihydrofolate reductase (DHFR). LR-73 cells, a line of Chinese hamster ovary cells, were transfected with this recombinant construct or with the parent vector, then subjected to culture in selective medium containing steadily increasing concentrations of methotrexate, a drug which specifically inhibits DHFR. Cultures transfected with the HSAG-1-containing construct acquired drug resistance faster than those transfected with the parent vector. This acceleration of acquisition of drug resistance was due to an increased probability of the generation and subsequent selection of cellular variants with increased copy numbers of the vector. The effect has also been observed in CHO(DHFR-) and HeLa cell lines. Possible mechanisms for the effect of the HSAG-1 element on gene amplification are discussed.

A mouse carcinoembryonic antigen gene family member is a calcium-dependent cell adhesion molecule

Carcinoembryonic antigen (CEA) is a heavily glycosylated protein used clinically as a tumor marker to detect recurrences of many types of tumors. This glycoprotein belongs to the immunoglobulin superfamily and is the prototype of the large CEA family of proteins. In a concerted effort to determine the function(s) of this family, we have been investigating a similar family of proteins in the mouse. In this paper, we report the characterization of a new mouse family member named mmCGM2; this gene product is highly homologous to the human biliary glycoprotein of the CEA gene family and to a rat hepatocyte ecto-ATPase. In vitro transcription, translation, and glycosylation experiments have revealed that the mmCGM2 cDNA encodes a glycoprotein of 42 kDA with a putative extracellular N-terminal domain and a C2-set type immunoglobulin domain. We have used this cDNA as a probe to detect many different transcripts (1.5-4.6 kilobases) in mouse adult tissues, some of which are specific to particular tissues, while others are expressed ubiquitously. After transfection of a plasmid bearing the mmCGM2 cDNA into mouse fibroblasts known to lack CEA-related gene expression, transfectant cell clones were chosen and used to investigate the adhesion properties conferred onto the cells. Cells expressing the mmCGM2 cDNA in a sense orientation aggregated in a calcium- and temperature-dependent fashion. Together with human biliary glycoprotein, the mmCGM2 gene product is the first member of the immunoglobulin superfamily to exhibit calcium-dependent adhesion. The constant tissue reorganization necessary to the differentiation of precise structures in tissues which express these gene family members (colon, liver, and uterus) implies the necessity of a variety of specific cell-cell contacts which could utilize the cell adhesion properties that we have demonstrated.

Stable, high-level expression of a carcinoembryonic antigen-encoding cDNA after transfection and amplification with the dominant and selectable asparagine synthetase marker

The introduction and expression of cloned genes in a wide variety of animal cells requires the convenient use of dominant selectable markers. Very few of these markers can be amplified in copy number, a necessary feature if variable and high-level expression of the gene of interest is required. We describe the successful dominant transfection and amplification of a vector containing the Escherichia coli asparagine synthetase (AS)-encoding gene, asnA, transfected into a variety of human and rodent cell lines. An unlinked co-transfected expression vector containing the CEA cDNA, encoding human carcinoembryonic antigen, can be co-amplified with the asnA marker leading to extremely high levels of CEA synthesis. In addition, we show that the expression of both the asnA marker and the co-transfected CEA construct are stable in normal and amplified transfectants after prolonged culture in the absence of selective pressure.

Biliary glycoprotein, a member of the immunoglobulin supergene family, functions in vitro as a Ca2(+)-dependent intercellular adhesion molecule

Intercellular adhesion molecules can be classified as Ca2+ dependent or Ca2+ independent. This classification has significant functional implications regarding cellular interactions. The best characterized Ca2(+)-dependent adhesion molecules, such as L-CAM or E-cadherin, belong to the family of closely related cell surface molecules called cadherins. On the other hand, those immunoglobulin supergene family members which function as adhesion molecules, such as neural cell adhesion molecule, have been found to be Ca2+ independent. In agreement with this generalization, we have recently shown that carcinoembryonic antigen (CEA) and nonspecific cross-reacting antigen (NCA), two closely related members of the CEA family, a subset of the immunoglobulin supergene family, function in vitro as Ca2(+)-independent adhesion molecules. In contrast, we show here that transfectants of a third member of the CEA family, biliary glycoprotein (BGP), also aggregate homotypically in suspension but require Ca2+ for aggregation. In addition, like the cadherins and unlike CEA or NCA or other adhesion molecules of the immunoglobulin supergene family, BGP transfectant aggregation requires physiological temperatures. Two forms of BGP, with three and two immunoglobulin C2-set domains, show Ca2(+)- and temperature-dependent adhesion, so that these properties do not reside in the third C2-set domain. The significance of this expression in the range of functional properties of the immunoglobulin supergene family and its CEA subset is discussed.

Specificity of intercellular adhesion mediated by various members of the immunoglobulin supergene family

The immunoglobulin supergene family members have been shown to be involved in cell-cell recognition and interaction during cell growth and differentiation. Neural cell adhesion molecule, myelin-associated glycoprotein, and carcinoembryonic antigen (CEA) are immunoglobulin supergene family members which can mediate cell adhesion. We show here that nonspecific cross-reacting antigen (NCA), a closely related CEA family member, is found on the surface of rodent cells transfected with functional NCA complementary DNA in different glycosylated forms, all of which can be deglycosylated to an Mr 35,000 core protein. Furthermore, NCA can mediate Ca2(+)-independent, homotypic aggregation of these NCA-producing transfectant cells. Since CEA has three internal repeated C2-set, immunoglobulin-like domains, whereas NCA has one, only one such domain is required for the intercellular adhesive function. We also demonstrate that NCA- and CEA-producing transfectants can form heterotypic aggregates, whereas mixtures of CEA or NCA transfectants and neural cell adhesion molecule or long form-myelin-associated glycoprotein transfectants sort themselves out into homotypic aggregates. The results suggest that subsets of the immunoglobulin superfamily, such as the CEA family, can be used in both homotypic and heterotypic cellular interactions, whereas less closely related members of the family can be used to separate different cell types by strictly homotypic interactions.