Host-related carcinoembryonic antigen cell adhesion molecule 1 promotes metastasis of colorectal cancer

Liver metastasis is the predominant cause of colorectal cancer (CRC)-related mortality in developed countries. Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is a cell adhesion molecule with reduced expression in early phases of CRC development and thus functions as a tumor growth inhibitor. However, CEACAM1 is upregulated in metastatic colon cancer, suggesting a bimodal role in CRC progression. To investigate the role of this protein in the host metastatic environment, Ceacam1(-/-) mice were injected intrasplenically with metastatic MC38 mouse CRC cells. A significant reduction in metastatic burden was observed in Ceacam1(-/-) compared with wild-type (WT) livers. Intravital microscopy showed decreased early survival of MC38 cells in Ceacam1(-/-) endothelial environment. Metastatic cell proliferation within the Ceacam1(-/-) livers was also diminished. Bone marrow-derived cell recruitment, attenuation of immune infiltrates and diminished CCL2, CCL3 and CCL5 chemokine production participated in the reduced Ceacam1(-/-) metastatic phenotype. Transplantations of WT bone marrow (BM) into Ceacam1(-/-) mice fully rescued metastatic development, whereas Ceacam1(-/-) BM transfer into WT mice showed reduced metastatic burden. Chimeric immune cell profiling revealed diminished recruitment of CD11b(+)Gr1(+) myeloid-derived suppressor cells (MDSCs) to Ceacam1(-/-) metastatic livers and adoptive transfer of MDSCs confirmed the involvement of these immune cells in reduction of liver metastasis. CEACAM1 may represent a novel metastatic CRC target for treatment.

Characterization of a major colon cancer susceptibility locus (Ccs3) on mouse chromosome 3

Treatment of mice with the carcinogen azoxymethane (AOM) induces a number of lesions in the colon, including hyperplastic lesions, as well adenomas and carcinomas in situ. Inbred strains of mice show different responses to AOM-induced carcinogenesis. A/J mice are highly susceptible and develop a greater number of hyperplastic lesions and tumors (15-70 tumors per mouse) than resistant C57BL/6J mice (0-6 tumors per mouse). Susceptibility to AOM-induced tumors segregates as a co-dominant trait in (A x B6)F1 hybrids. Using a set of 23 AcB and BcA recombinant congenic mouse strains derived from A/J (susceptible) and B6 (resistant) parents, we observed that the number of hyperplastic lesions and tumors induced by AOM was under different genetic controls in AcB/BcA strains. The multiplicity of AOM-induced tumors is controlled by a major locus that we have mapped on the distal portion of chromosome 3, to which we have given the temporary designation colon cancer susceptibility locus 3 (Ccs3). B6 and A/J alleles at Ccs3 are associated with resistance and susceptibility, respectively. Haplotype analysis in key informative AcB/BcA strains restricts the size of the Ccs3 locus to a 14 Mb segment that contains 94 annotated genes. The expression level of all these genes in normal colon has been established by transcript profiling with microarrays, and has led to the identification of a subset of positional candidates that are expressed at high levels in this tissue. The 4q and 1p human chromosomal segments sharing syntenic homology with the mouse Ccs3 segment are known to be associated with inflammatory bowel diseases and colorectal tumors in humans, suggesting that the study of the mouse Ccs3 locus may help further the pathogenesis of these human conditions.

Deletion of the carcinoembryonic antigen-related cell adhesion molecule 1 (Ceacam1) gene contributes to colon tumor progression in a murine model of carcinogenesis

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is a glycoprotein that is part of the carcinoembryonic antigen and the immunoglobulin superfamilies. We have shown that it functions as a tumor suppressor and that this function depends upon the presence of the longer CEACAM1 cytoplasmic domain. In this report, we describe the generation of a Ceacam1-/- mouse. The Ceacam1-/- colon exhibits increased in vivo proliferation relative to the wild-type counterpart with a corresponding decreased expression of the p21(Cip1) and p27(Kip1) Cyclin D kinase inhibitors. The colonic villi undergo decreased apoptosis. Out of 35 litters of mice, no spontaneous tumors in any tissues normally expressing CEACAM1 were found over the lifespan of the animals, suggesting that CEACAM1 may not be involved in initiation of tumor development. However, when mice are treated with azoxymethane to induce colonic tumors, we find that Ceacam1-/- mice developed a significantly greater number of tumors than their littermate controls. Moreover, the tumor size was greater in the knockout mice relative to that in the wild-type mice. These results indicate that deletion of CEACAM1 favors progression of colon tumorigenesis.

Targeted disruption of the Ceacam1 (MHVR) gene leads to reduced susceptibility of mice to mouse hepatitis virus infection

The CEACAM1 glycoproteins (formerly called biliary glycoproteins; BGP, C-CAM, CD66a, or MHVR) are members of the carcinoembryonic antigen family of cell adhesion molecules. In the mouse, splice variants of CEACAM1 have either two or four immunoglobulin (Ig) domains linked through a transmembrane domain to either a short or a long cytoplasmic tail. CEACAM1 has cell adhesion activity and acts as a signaling molecule, and long-tail isoforms inhibit the growth of colon and prostate tumor cells in rodents. CEACAM1 isoforms serve as receptors for several viral and bacterial pathogens, including the murine coronavirus mouse hepatitis virus (MHV) and Haemophilus influenzae, Neisseria gonorrhoeae, and Neisseria meningitidis in humans. To elucidate the mechanisms responsible for the many biological activities of CEACAM1, we modified the expression of the mouse Ceacam1 gene in vivo. Manipulation of the Ceacam1 gene in mouse embryonic stem cells that contained the Ceacam1a allele yielded a partial knockout. We obtained one line of mice in which the insert in the Ceacam1a gene had sustained a recombination event. This resulted in the markedly reduced expression of the two CEACAM1a isoforms with four Ig domains, whereas the expression of the two isoforms with two Ig domains was doubled relative to that in wild-type BALB/c (+/+) mice. Homozygous (p/p) Ceacam1a-targeted mice (Ceacam1aDelta4D) had no gross tissue abnormalities and were viable and fertile; however, they were more resistant to MHV A59 infection and death than normal (+/+) mice. Following intranasal inoculation with MHV A59, p/p mice developed markedly fewer and smaller lesions in the liver than +/+ or heterozygous (+/p) mice. The titers of virus produced in the livers were 50- to 100-fold lower in p/p mice than in +/p or +/+ mice. p/p mice survived a dose 100-fold higher than the lethal dose of virus for +/+ mice. +/p mice were intermediate between +/+ and p/p mice in susceptibility to liver damage, virus growth in liver, and susceptibility to killing by MHV. Ceacam1a-targeted mice provide a new model to study the effects of modulation of receptor expression on susceptibility to MHV infection in vivo.

The CEACAM1-L Ser503 residue is crucial for inhibition of colon cancer cell tumorigenicity

CEACAM1 (also known as biliary glycoprotein, C-CAM or CD66a) is a cell adhesion molecule of the immunoglobulin family behaving as a tumor inhibitory protein in colon, prostate, liver, endometrial and breast cancers. Inhibition of tumor development is dependent upon the presence of the long 71-73 amino acid cytoplasmic domain of the CEACAM1 protein (CEACAM1-L). We have recently defined a number of cis-acting motifs within the long cytoplasmic domain participating in tumor cell growth inhibition. These are Tyr488, corresponding to an Immunoreceptor Tyrosine-based Inhibition Motif, as well as the three terminal lysine residues of the protein. In this study, we provide evidence that treatment with phorbol esters leads to increased phosphorylation of in vivo (32)P-labeled CEACAM1-L in mouse CT51 carcinoma cells, in the mouse 1MEA 7R.1 liver carcinoma cells and in 293 human embryonic kidney cells transfected with the Ceacam1-L cDNA. Basal level Ser phosphorylation was abrogated by treatment with the staurosporine inhibitor, but not by the protein kinase C-specific inhibitor calphostin C or other inhibitors such as H7 or sphingosine. Specific inhibitors of protein kinase A or calmodulin kinase had only minimal effects on the levels of basal or PMA-induced Ser phosphorylation. Furthermore, PMA treatment of the CT51 cells induced cell spreading and cellular relocalization of the CEACAM1-L protein. Since Ser503 has been described as a PMA-induced phosphorylation site in other cell systems, we investigated whether Ser503 was involved in these responses in mouse intestinal cells. No differences were noticed in the basal or the PMA-induced phosphorylation levels, kinase inhibitor sensitivity or the PMA-induced relocalization of the protein between the wild-type and the Ser503Ala mutant CEACAM1-L. However, we provide evidence that Ser503 participates in CEACAM1-L-mediated tumor inhibition as its mutation to an Ala led to in vivo tumor development, contrary to the tumor inhibitory phenotype observed with the wild-type CEACAM1-L protein.

cis-Determinants in the cytoplasmic domain of CEACAM1 responsible for its tumor inhibitory function

CEACAM1, also known as C-CAM, BGP and CD66a, is a member of the carcinoembryonic antigen (CEA) family which is itself part of the immunoglobulin supergene family. CEACAM1 is involved in intercellular adhesion, signal transduction and tumor cell growth regulation. CEACAM1 is down-regulated in colon and prostate carcinomas, as well as in endometrial, bladder and hepatic tumors, and 30% of breast cancers. We have shown in a mouse colon tumor model that CEACAM1 with a long cytoplasmic domain inhibited the development of tumors whereas a splice variant lacking the cytoplasmic domain did not. In this study, we define the subregions of the long cytoplasmic domain participating in the tumor inhibition phenotype of CEACAM1. We show that a single point mutation of Tyr488, conforming to an Immunoreceptor Tyrosine Inhibition Motif (ITIM), was sufficient to reverse the in vivo tumor cell growth inhibition. Substitution or deletion of residues in the C-terminal region of the CEACAM1 cytoplasmic domain also led to reversal of tumor cell growth inhibition. This result is in agreement with our previous studies demonstrating the C-terminal region of the cytoplasmic domain influences the levels of CEACAM1 Tyr phosphorylation and its association with the protein Tyr phosphatases SHP-1 and SHP-2. Furthermore, removal of the N-terminal domain of CEACAM1, essential for intercellular adhesion, did not impair the tumor inhibitory effect. These results suggest that Tyr phosphorylation or dephosphorylation of the CEACAM1 cytoplasmic domain represents a crucial step in the control of epithelial cell proliferation.

Optimal ratios of biliary glycoprotein isoforms required for inhibition of colonic tumor cell growth

Rodent biliary glycoprotein (Bgp), also known as C-CAM, has recently been shown to function as a tumor suppressor in colon, prostate, and bladder cancers. This glycoprotein is a member of the carcinoembryonic antigen family and is one of the only proteins in this family to encode either a long (71-73 amino acids) or short (10 amino acids) cytoplasmic domain. We and others have shown that the growth-inhibitory properties of Bgp depend upon the expression of its long cytoplasmic domain. However, the two Bgp isoforms normally coexist in most cell types surveyed; the longer variant is usually present in lower amounts than the shorter one. In this study, we have examined the in vitro and in vivo growth properties of both mouse Bgp variants separately and in combination. To determine the physiologically relevant expression levels and ratios of the two Bgp variants, we have quantified the amount of the longer variant in normal colonic epithelial cells and showed that it constitutes 15-20% of total Bgp expressed in this tissue. To mimic the in vivo situation, we have generated double transfectant cell lines expressing the longer and shorter Bgp isoforms coordinately in tumorigenic CT51 mouse colonic carcinoma cells and demonstrated that the longer Bgp isoform exhibits a dominant tumor growth inhibition phenotype over that of the shorter variant within physiological levels of expression of Bgp. Unexpectedly, significant overexpression of the longer Bgp isoform alone led to reversal of the tumor inhibition phenotype. These results, therefore, suggest that there may be a limiting threshold of Bgp expression or Bgp-associating proteins mediating the tumor inhibition phenotype.

Association of biliary glycoprotein with protein tyrosine phosphatase SHP-1 in malignant colon epithelial cells

Biliary glycoprotein (Bgp) is a member of the immunoglobulin superfamily and the carcinoembryonic antigen family. Previous studies have shown that Bgp functions as an intercellular adhesion molecule and a canalicular bile salt transporter. Moreover, we and others demonstrated that Bgp can inhibit colonic and prostatic tumor cell growth in vivo, through a mechanism which depends on sequences present in its cytoplasmic domain. In this study, we have examined the possibility that the cytoplasmic domain of Bgp can interact with signal transduction molecules. We showed that tyrosine phosphorylated Bgp, expressed in mouse colon carcinoma CT51 cells, could reversibly associate with protein tyrosine phosphatase SHP-1. Mutation of either of two tyrosine residues present in the cytoplasmic domain of Bgp abrogated SHP-1 binding, suggesting that this association was mediated by both tyrosine residues. Similarly, we noted that either of the two SH2 domains of SHP-1 could bind tyrosine phosphorylated Bgp in vitro. It is therefore conceivable that some of the functions of Bgp are mediated through its ability to induce intracellular protein tyrosine dephosphorylation.

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.

Inhibition of colonic tumor cell growth by biliary glycoprotein

Biliary glycoproteins (BGPs) are members of the carcinoembryonic antigen (CEA) family. These glycoproteins function in vitro as intercellular adhesion molecules and, in vitro as intercellular adhesion molecules and, in the mouse, serve as receptors for the mouse hepatitis viruses. In previous studies, BGP expression has been reported to be generally downregulated in colon and liver carcinomas of human, rat and mouse origins. We now demonstrate that introduction of murine Bgp1 cDNA isoforms into a mouse colonic carcinoma cell line, negative for endogenous Bgpl expression, significantly alters the growth properties of these cells. Cells bearing two Bgp1 isoforms were growth-retarded and exhibited a reduced ability to form colonies in an in vitro transformation assay, when compared to parental or control neor cells. Furthermore, tumor formation was inhibited by 80% when cells bearing a full-length Bgp1 isoform were injected into BALB/c syngeneic mice, while cells expressing a Bgp1 isoform lacking most of the intracytoplasmic domain produced tumors as readily as the parental cells. There results indicate that a biliary glycoprotein isoform is involved in negative regulation of colonic tumor cell growth, by a process which requires its intracytoplasmic domain. The precise mechanisms causing Bgp-dependent tumor growth inhibition remain, however, to be defined.

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.

The expression of mouse biliary glycoprotein, a carcinoembryonic antigen-related gene, is down-regulated in malignant mouse tissues

Mouse biliary glycoprotein (Bgp) is a member of the carcinoembryonic antigen gene family and is highly expressed in the epithelial cells of normal hepatic biliary ducts and intestine. Nine mouse Bgp isoforms have been identified through molecular cloning and shown to be splice and allelic variants of one Bgp gene. These glycoproteins function in vitro as intercellular adhesion molecules and serve as the mouse hepatitis virus receptors. Since human carcinoembryonic antigen is overexpressed in gastrointestinal tumors, we have investigated the expression of mouse Bgp in primary tumors and carcinoma cell lines. Our results demonstrate that the expression of the major mouse Bgp isoforms is down-regulated in tumors at the transcriptional and the posttranscriptional levels. This decrease in expression is corroborated by immunostaining of primary colonic tumors with anti-mouse Bgp antibodies. In addition, Bgp expression is influenced by transcriptional control mechanisms involving DNA methylation of the Bgp gene upstream regulatory region. Our results demonstrate that mouse Bgp protein expression is decreased upon malignant transformation and further suggest that Bgp proteins may be involved in the maintenance of the differentiated cellular phenotype.

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.

mmCGM1a: a mouse carcinoembryonic antigen gene family member, generated by alternative splicing, functions as an adhesion molecule

Carcinoembryonic antigen is a human tumor marker and the prototype of a large family of immunoglobulin-like proteins. We have been developing a mouse model for this large protein family and have cloned a complementary DNA (cDNA) for a mouse carcinoembryonic antigen gene family member (mmCGM1a). Two transcripts expressed in several different adult mouse tissues hybridize to this cDNA, a 1.8-kilobase and a 4.6-kilobase mRNA. Sequences of many related cDNA clones indicate that they are most likely encoded by a single gene which undergoes alternative splicing. The protein encoded by the mmCGM1a cDNA shares 69% of the amino acid residues in the NH2-terminal domain with a rat liver ecto-ATPase and with the human biliary glycoprotein. Mouse fibroblast transfectant cells expressing the mmCGM1a protein on their cell surface exhibit calcium- and temperature-independent adhesion in vitro which can be specifically inhibited by an antibody raised against a carcinoembryonic antigen-related 120 kilodalton protein.

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.

Spatiotemporal expression of murine carcinoembryonic antigen (CEA) gene family members during mouse embryogenesis

Carcinoembryonic antigen is a glycosylated protein used as a human tumor marker to assess recurrences of gastrointestinal, breast and lung cancers. This protein is a member of the immunoglobulin supergene family and has been shown to function as a homophilic and heterophilic cell adhesion molecule. We have studied the spatial and temporal expression of two cloned mouse CEA gene family members during development using Northern analyses and in situ hybridization. Transcripts detected by the mouse CEA probes are expressed from 10.5 days post coitum (p.c.) to birth in mouse embryos and fetuses and are present from 16.5 days p.c. to adulthood in intestine and colon as evaluated by Northern analyses. The RNA is also present in many other tissues including meninges, cartilage and bone, blood vessel walls, placenta, dermis, muscle layers of the stomach and intestine and bronchioles of developing mouse embryos and fetuses. This expression pattern is similar to that of proteins of the Transforming Growth Factor beta gene family. The role of mouse CEA gene family members is unclear; however, the localizations of these CEA family members in the mouse embryo and fetus suggest an important functional role during active morphogenesis, a complex process in which cell adhesion molecules are significantly involved.

A mouse analogue of the human carcinoembryonic antigen

Functional human carcinoembryonic antigen (CEA)-like genes have been shown to be present in the mouse. Southern analyses of murine DNA using both human and murine CEA complementary DNA probes have revealed the presence of multiple CEA-like genes, while analyses of RNA from different mouse tissues showed CEA-like transcripts in adult colon and liver. Furthermore, a CEA-like protein, immunoprecipitable with a rabbit polyclonal serum raised against human CEA, has been detected in adult murine colon tissue. Several murine CEA complementary DNA clones have been isolated from a murine colon complementary DNA library, and characterization of one such clone demonstrates that both the N-terminal and the internal domains have been conserved between the two species. The existence of a murine counterpart of CEA strengthens the case for an essential function for this human tumor marker and provides an experimentally amenable system for elucidation of its biological properties.

Incorporation of myristate and palmitate into the sheep reticulocyte transferrin receptor: evidence for identical sites of labeling

The ability of sheep reticulocytes and plasma membranes isolated from them to incorporate fatty acids into the transferrin receptor has been examined using both [3H]palmitate and [3H]myristate. Both fatty acids, when incorporated into the transferrin receptor, can be released by treating the protein with 1 M hydroxylamine at pH 7.0. After treatment of the 3H-acylated receptor with borohydride, an 3H-labeled alcohol is released, suggesting that the receptor-bound fatty acid is in thioester linkage. With both [3H]myristate and [3H]palmitate, Cleveland maps from immunoprecipitates of the transferrin receptor labeled in intact cells and isolated membranes show that identical peptides are labeled. No evidence was obtained for qualitatively different labeling with the two fatty acids. In intact reticulocytes, incorporation of [3H]palmitate into the transferrin receptor is approximately 3.5 times greater than the incorporation of [3H]myristate from equivalent concentrations of the labeled fatty acids. However, in isolated reticulocyte plasma membranes, there is much less difference between palmitate and myristate incorporation (with ATP) or between their acyl-CoA derivatives. The reason for the discrepancy between cells and membranes is unknown but may be due to the presence in intact cells of more than one enzyme for activating the fatty acids. Acylation of the receptor in isolated plasma membranes is fourfold greater with the CoA derivatives than with the free fatty acids. The fatty acid activating enzyme(s) as well as the acyltransferase(s) appear to be membrane bound in reticulocytes.

Vesicle formation during reticulocyte maturation. Association of plasma membrane activities with released vesicles (exosomes)

Vesicles are released during the in vitro culture of sheep reticulocytes which can be harvested by centrifugation at 100,000 X g for 90 min. These vesicles contain a number of activities, characteristic of the reticulocyte plasma membrane, which are known to diminish or disappear upon reticulocyte maturation. The activities include acetylcholinesterase, cytochalasin B binding (glucose transporter) nucleoside binding (i.e. nucleoside transporter), Na+-independent amino acid transport, and the transferrin receptor. Enzymes of cytosolic origin are not detectable or are present at low activity in the vesicles. Cultures of whole blood, mature red cells, or white cells do not yield comparable levels of these activities, supporting the conclusion that the activities arise from the reticulocytes. In addition, the lipid composition of the vesicles shows the high sphingomyelin content characteristic of sheep red cell plasma membranes, but not white cell or platelet membranes, also consistent with the conclusion that the vesicles are of reticulocyte origin. It is suggested that vesicle externalization may be a mechanism for shedding of specific membrane functions which are known to diminish during maturation of reticulocytes to erythrocytes.

Evidence for a pool of non-recycling transferrin receptors in peripheral sheep reticulocytes

Sheep reticulocytes from phlebotomized animals have a total transferrin binding potential that may exceed by an order of magnitude the surface binding capacity. Steady state uptake of transferrin at 37 degrees C is generally less than 50% of the total transferrin binding capacity. During long-term incubation of the reticulocytes, all transferrin binding ability is lost, the ability to internalize being lost most rapidly. The loss in ability to bind transferrin during long-term incubation is independent of the number of surface transferrin binding sites, since removal of surface receptors with pronase does not affect the rate of loss of the internal pool of receptors during long-term incubation. Moreover, after removing surface receptors with pronase, only a fraction of the original number of receptors is restored to the surface, despite the presence of a large pool of internal receptors. These data suggest that only a fraction of the internal pool of receptors is capable of recycling to the cell surface in sheep reticulocytes.

In vitro acylation of the transferrin receptor

In vitro fatty acylation of the transferrin receptor with [3H]tetradecanoate or [3H]tetradecanoyl-CoA has been demonstrated for isolated sheep reticulocyte plasma membranes. Although less than 5% of the receptor was labeled in vitro, the acylated protein could be readily observed after sodium dodecyl sulfate-gel electrophoresis. The acylated transferrin receptor in the reticulocyte membrane was specifically precipitated with a monoclonal antibody and was absent from mature red cell membranes. Incorporation of fatty acid was dependent on ATP, and fatty acid was 5-10 times less effective as an acyl donor than the acyl-CoA derivative, pointing out the strong potential of this reagent for in vitro acylation of membrane proteins. During in vitro maturation of reticulocytes, the receptor is released in vesicles into the incubation medium. Using reticulocytes labeled with [3H]tetradecanoate, it can be shown that the 3H-labeled receptor is transferred from the cells to the vesicles without loss of acyl groups, suggesting that the vesiculation process does not involve deacylation.

Phosphorylation of the transferrin receptor in isolated sheep reticulocyte plasma membranes

The transferrin receptor of sheep reticulocyte plasma membranes undergoes phosphorylation with [gamma-32P]ATP in isolated plasma membranes. The phosphorylation is stimulated by Mn2+, Co2+, and Mg2+, but not by Ca2+, Ba2+, Zn2+, Fe2+, or Cu2+. There is no detectable effect of cyclic nucleotides on the phosphorylation of the receptor. Transferrin and a monoclonal antibody against the transferrin receptor have no apparent effect on receptor phosphorylation in intact cells or isolated membranes. Immunoprecipitates of the receptor retain ability to phosphorylate the receptor. The phosphorylation appears to be at a serine residue which turns over with a half time of 20-30 min. ATP appears to be the best, but not the only substrate for receptor phosphorylation.