Differential regulation of C-CAM isoforms in epithelial cells

Size Matters: The Functional Role of the CEACAM1 Isoform Signature and Its Impact for NK Cell-Mediated Killing in Melanoma

Malignant melanoma is the most aggressive and treatment resistant type of skin cancer. It is characterized by continuously rising incidence and high mortality rate due to its high metastatic potential. Various types of cell adhesion molecules have been implicated in tumor progression in melanoma. One of these, the carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), is a multi-functional receptor protein potentially expressed in epithelia, endothelia, and leukocytes. CEACAM1 often appears in four isoforms differing in the length of their extracellular and intracellular domains. Both the CEACAM1 expression in general, and the ratio of the expressed CEACAM1 splice variants appear very dynamic. They depend on both the cell activation stage and the cell growth phase. Interestingly, normal melanocytes are negative for CEACAM1, while melanomas often show high expression. As a cell–cell communication molecule, CEACAM1 mediates the direct interaction between tumor and immune cells. In the tumor cell this interaction leads to functional inhibitions, and indirectly to decreased cancer cell immunogenicity by down-regulation of ligands of the NKG2D receptor. On natural killer (NK) cells it inhibits NKG2D-mediated cytolysis and signaling. This review focuses on novel mechanistic insights into CEACAM1 isoforms for NK cell-mediated immune escape mechanisms in melanoma, and their clinical relevance in patients suffering from malignant melanoma.

Basal cells of the human airways acquire mesenchymal traits in idiopathic pulmonary fibrosis and in culture

Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease with high morbidity and mortality. The cellular source of the fibrotic process is currently under debate with one suggested mechanism being epithelial-to-mesenchymal transition (EMT) in the alveolar region. In this study, we show that airway epithelium overlying fibroblastic foci in IPF contains a layer of p63-positive basal cells while lacking ciliated and goblet cells. This basal epithelium shows increased expression of CK14, Vimentin and N-cadherin while retaining E-cadherin. The underlying fibroblastic foci shows both E- and N-cadherin-positive cells. To determine if p63-positive basal cells were able to undergo EMT in culture, we treated VA10, a p63-positive basal cell line, with the serum replacement UltroserG. A sub-population of treated cells acquired a mesenchymal phenotype, including an E- to N-cadherin switch. After isolation, these cells portrayed a phenotype presenting major hallmarks of EMT (loss of epithelial markers, gain of mesenchymal markers, increased migration and anchorage-independent growth). This phenotypic switch was prevented in p63 knockdown (KD) cells. In conclusion, we show that airway epithelium overlying fibroblastic foci in IPF lacks its characteristic functional identity, shows increased reactivity of basal cells and acquisition of a partial EMT phenotype. This study suggests that some p63-positive basal cells are prone to phenotypic changes and could act as EMT progenitors in IPF.

CEACAM1 functionally interacts with filamin A and exerts a dual role in the regulation of cell migration

The carcinoembryonic antigen-related cell adhesion molecule CEACAM1 (CD66a) and the scaffolding protein filamin A have both been implicated in tumor cell migration. In the present study we identified filamin A as a novel binding partner for the CEACAM1-L cytoplasmic domain in a yeast two-hybrid screen. Direct binding was shown by surface plasmon resonance analysis and by affinity precipitation assays. The association was shown for human and rodent CEACAM1-L in endogenous CEACAM1-L expressing cells. To address functional aspects of the interaction, we used a well-established melanoma cell system. We found in different migration studies that the interaction of CEACAM1-L and filamin A drastically reduced migration and cell scattering, whereas each of these proteins when expressed alone, acted promigratory. CEACAM1-L binding to filamin A reduced the interaction of the latter with RalA, a member of the Ras-family of GTPases. Furthermore, co-expression of CEACAM1-L and filamin A led to a reduced focal adhesion turnover. Independent of the presence of filamin A, the expression of CEACAM1-L led to an increased phosphorylation of focal adhesions and to altered cytoskeletal rearrangements during monolayer wound healing assays. Together, our data demonstrate a novel mechanism for how CEACAM1-L regulates cell migration via its interaction with filamin A.

Control of density-dependent, cell state-specific signal transduction by the cell adhesion molecule CEACAM1, and its influence on cell cycle regulation

Growth factor receptors, extracellular matrix receptors, and cell-cell adhesion molecules co-operate in regulating the activities of intracellular signaling pathways. Here, we demonstrate that the cell adhesion molecule CEACAM1 co-regulates growth-factor-induced DNA synthesis in NBT-II epithelial cells in a cell-density-dependent manner. CEACAM1 exerted its effects by regulating the activity of the Erk 1/2 MAP kinase pathway and the expression levels of the cyclin-dependent kinase inhibitor p27(Kip1). Interestingly, both inhibitory and stimulatory effects were observed. Confluent cells continuously exposed to fetal calf serum showed little Erk activity and DNA synthesis compared with sparse cells. Under these conditions, anti-CEACAM1 antibodies strongly stimulated Erk activation, decreased p27 expression, and induced DNA synthesis. In serum-starved confluent cells, re-addition of 10% fetal calf serum activated the Erk pathway, decreased p27 expression, and stimulated DNA synthesis to the same levels as in sparse cells. Under these conditions anti-CEACAM1 antibodies de-activated Erk, restored the level of p27, and inhibited DNA synthesis. These data indicate that CEACAM1 mediates contact inhibition of proliferation in cells that are constantly exposed to growth factors, but co-activates growth-factor-induced proliferation in cells that have been starved for growth factors; exposure to extracellular CEACAM1 ligands reverts these responses.

Transmembrane CEACAM1 affects integrin-dependent signaling and regulates extracellular matrix protein-specific morphology and migration of endothelial cells

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1/CD66a), expressed on leukocytes, epithelia, and endothelia mediates homophilic cell adhesion. It plays an important role in cell morphogenesis and, recently, soluble CEACAM1 isoforms have been implicated in angiogenesis. In the present study, we investigated the function of long transmembrane isoform of CEACAM1 (CEACAM1-L) in cultured rat brain endothelial cells. We observed that expression of CEACAM1-L promotes network formation on basement membrane Matrigel and increased cell motility after monolayer injury. During cell-matrix adhesion, CEACAM1-L translocated into the Triton X-100-insoluble cytoskeletal fraction and affected cell spreading and cell morphology on Matrigel and laminin-1 but not on fibronectin. On laminin-1, CEACAM1-L-expressing cells developed protrusions with lamellipodia, showed less stress fiber formation, reduced focal adhesion kinase (FAK) tyrosine phosphorylation, and decreased focal adhesion formation leading to high motility. CEACAM1-L-mediated morphologic alterations were sensitive to RhoA activation via lysophosphatidic acid (LPA) treatment and dependent on Rac1 activation. Furthermore, we demonstrate a matrix protein-dependent association of CEACAM1-L with talin, an important regulator of integrin function. Taken together, our results suggest that transmembrane CEACAM1-L expressed on endothelial cells is implicated in the activation phase of angiogenesis by affecting the cytoskeleton architecture and integrin-mediated signaling.

The cytoplasmic domain of CEACAM1-L controls its lateral localization and the organization of desmosomes in polarized epithelial cells

Two CEACAM1 isoforms with different cytoplasmic domains, CEACAM1-L and CEACAM1-S, are unequally distributed in polarized epithelial MDCK cells. CEACAM1-S is exclusively apical whereas CEACAM1-L occurs both in apical and lateral cell surfaces. Using confocal microscopy and CEACAM1-L mutants, we identified several amino acids in the cytoplasmic domain that were instrumental for the lateral localization. Tyr515, but not Tyr488, constituted a prominent lateral targeting signal. Pervanadate-stimulated Tyr phosphorylation induced rapid phosphatidylinositol 3-kinase-dependent disappearance of lateral CEACAM1-L, whereas staurosporine, a Ser/Thr kinase inhibitor, resulted in slower phosphatidylinositol 3-kinase-independent disappearance. Both drugs caused accumulation of CEACAM1-L in a late endosome/lysosome compartment. Colocalization studies of occludin, ZO-1, E-cadherin, beta-catenin and desmoplakin indicated that laterally localized CEACAM1-L was present in adherens junctions but not in tight junctions or desmosomes. Overexpressed CEACAM1-L did not affect the organization of tight junction or adherens junction proteins, but perturbed the arrangement of desmosomes. The abundance of desmosomes in the lateral cell surfaces decreased significantly and the submembraneous cytokeratin filaments became disorganized. The signal for desmosomal perturbance resided within amino acids 484-518 in the C-terminal part of the cytoplasmic domain, among which an intact Tyr515 was indispensable.

CEACAM1 isoforms with different cytoplasmic domains show different localization, organization and adhesive properties in polarized epithelial cells

CEACAM1 is a signaling cell adhesion molecule expressed in epithelia,vessel endothelia and leukocytes. It is expressed as two major isoforms with different cytoplasmic domains. CEACAM1 occurs both in cell-cell contact areas and on apical surfaces of polarized epithelial cells, but it is not known how the different isoforms are distributed in polarized cells or what the functions of CEACAM1 are in the apical surfaces. We investigated the localization and organization of the two CEACAM1 isoforms in transfected,polarized MDCK cells by confocal microscopy and differential surface labelling. CEACAM1-L was found on both the apical and the lateral surfaces,whereas CEACAM1-S appeared exclusively on the apical surfaces. Maintenance of the lateral localization of CEACAM1-L required homophilic binding between CEACAM1-L molecules on adjacent cells. Double-labelling with anti-CEACAM1 antibodies directed against different epitopes indicated that apical CEACAM1-L occurred either in a homophilic adhesive state or in a free non-adhesive state. CEACAM1-S appeared almost exclusively in the homophilic adhesive state. These findings suggest that CEACAM1 mediates adhesive bonds between adjacent microvilli on the apical surfaces.

The characteristic cellular organization and CEACAM1 expression in the junctional epithelium of rats and mice are genetically programmed and not influenced by the bacterial microflora

BACKGROUND

The epithelial cell adhesion molecule CEACAM1 exhibits an interesting dynamic expression during tooth development. It is first expressed in the reduced enamel epithelium, its expression then increases in the orally faced reduced epithelium and the overlying oral epithelium that then fuse to give rise to the junctional epithelium. The expression of CEACAM1 remains at high levels in the junctional epithelium, in contrast to the surrounding oral sulcular epithelium which shows much lower expression levels. We investigated if the high expression levels of CEACAM1 and the loosely organized cells characteristic of the junctional epithelium are genetically programmed or result from bacterial infiltration.

METHODS

Oral tissues from germ-free rats and mice and animals with conventional bacterial flora were analyzed by transmission electron microscopy and immunohistochemical staining for CEACAM1.

RESULTS

The junctional epithelium of both germ-free and conventional animals was identical with respect to both CEACAM1 expression and morphology. Also the presence of leukocytes was the same in both types of animals.

CONCLUSIONS

The results indicate that the characteristic morphology and the high expression levels of CEACAM1 in the junctional epithelium are genetically programmed and not a result of bacterial infiltration. This suggests that CEACAM1 has an important role for the structural integrity of the junctional epithelium. This conclusion was supported by the observation that the junctional epithelium does not express any E-cadherin, which is another abundant epithelial cell adhesion molecule.

The CEACAM1-L glycoprotein associates with the actin cytoskeleton and localizes to cell-cell contact through activation of Rho-like GTPases

Associations between plasma membrane-linked proteins and the actin cytoskeleton play a crucial role in defining cell shape and determination, ensuring cell motility and facilitating cell-cell or cell-substratum adhesion. Here, we present evidence that CEACAM1-L, a cell adhesion molecule of the carcinoembryonic antigen family, is associated with the actin cytoskeleton. We have delineated the regions involved in actin cytoskeleton association to the distal end of the CEACAM1-L long cytoplasmic domain. We have demonstrated that CEACAM1-S, an isoform of CEACAM1 with a truncated cytoplasmic domain, does not interact with the actin cytoskeleton. In addition, a major difference in subcellular localization of the two CEACAM1 isoforms was observed. Furthermore, we have established that the localization of CEACAM1-L at cell-cell boundaries is regulated by the Rho family of GTPases. The retention of the protein at the sites of intercellular contacts critically depends on homophilic CEACAM1-CEACAM1 interactions and association with the actin cytoskeleton. Our results provide new evidence on how the Rho family of GTPases can control cell adhesion: by directing an adhesion molecule to its proper cellular destination. In addition, these results provide an insight into the mechanisms of why CEACAM1-L, but not CEACAM1-S, functions as a tumor cell growth inhibitor.

Comparison of expression patterns and cell adhesion properties of the mouse biliary glycoproteins Bbgp1 and Bbgp2

Biliary glycoproteins are members of the carcinoembryonic antigen (CEA) family and behave as cell adhesion molecules. The mouse genome contains two very similar Bgp genes, Bgp1 and Bgp2, whereas the human and rat genomes contain only one BGP gene. A Bgp2 isoform was previously identified as an alternative receptor for the mouse coronavirus mouse hepatitis virus. This isoform consists of two extracellular immunoglobulin domains, a transmembrane domain and a cytoplasmic tail of five amino acids. In this report, we have examined whether the Bgp2 gene can express other isoforms in different mouse tissues. We found only one other isoform, which has a long cytoplasmic tail of 73 amino acids. The long cytodomain of the Bgp2 protein is highly similar to that of the Bgp1/4L isoform. The Bgp2 protein is expressed in low amounts in kidney and in a rectal carcinoma cell line. Antibodies specific to Bgp2 detected a 42-kDa protein, which is expressed at the cell surface of these samples. Bgp2 was found by immunocytochemistry in smooth muscle layers of the kidney, the uterus, in gut mononuclear cells and in the crypt epithelia of intestinal tissues. Transfection studies showed that, in contrast with Bgp1, the Bgp2 glycoprotein was not directly involved in intercellular adhesion. However, this protein is found in the proliferative compartment of the intestinal crypts and in cells involved in immune recognition. This suggests that the Bgp2 protein represents a distinctive member of the CEA family; its unusual expression patterns in mouse tissues and the unique functions it may be fulfilling may provide novel clues about the multiple functions mediated by a common BGP protein in humans and rats.

C-CAM-mediated adhesion leads to an outside-in dephosphorylation signal

The rat cell-cell adhesion molecule C-CAM, a member of the carcinoembryonic antigen family, was shown to be expressed in various isoforms, differing in the length of the cytoplasmic domain. The long isoform C-CAML inhibits the growth of different malignant cells. Several studies suggest that it is involved in the mechanism of signal transduction. So far no direct correlation between C-CAM function and C-CAM phosphorylation has been reported. In the present study we addressed the question of whether C-CAM-mediated adhesion is accompanied by changes in phosphorylation of the cytoplasmic domain of C-CAM. It was demonstrated that C-CAML is constitutively phosphorylated in adherent growing cells as well as in cells growing in suspension. In contrast, C-CAML-mediated cell aggregation is accompanied by a 40% reduction in C-CAML phosphorylation compared with nonaggregated cells. The same dephosphorylation was achieved by antibody-induced clustering of C-CAML in the plasma membrane. Phosphorylation and dephosphorylation indicate a C-CAM-mediated outside-in signalling induced by cell-cell adhesion.

The structural analysis of adhesions mediated by Ep-CAM

The epithelial cell adhesion molecule Ep-CAM is capable of mediating Ca2+-independent homotypic cell-cell adhesion when introduced into cells lacking their own means of cell-cell interactions. We used (confocal) immunofluorescent and (immuno-) electron microscopy to investigate the structural organization of Ep-CAM-mediated adhesions and their relation to other types of intercellular adhesions. Ep-CAM-transfected cell lines, cells of epithelial origin, and epithelial tissues were analyzed. In transfected L cells Ep-CAM brings the opposing intercellular membranes into a close proximity (approximately 10-14 nm) at sporadic contacts; however, no structures resembling junctional complexes were observed. In L cells cotransfected with Ep-CAM and E-cadherin, both molecules localize at the sites of cell-cell contact, forming independent adhesion sites with no Ep-CAM detectable within the structurally distinguishable cadherin-mediated adherens junctions. In well-differentiated carcinoma cell lines Ep-CAM colocalized with E-cadherin practically along the whole lateral domain; however, no colocalization was observed between Ep-CAM and the components of the tight junction complex (occludin and ZO-1), desmosomes (desmoplakins I/II), or cell-substrate adhesions (beta1 integrins). This was confirmed by analysis of polarized epithelium of normal colon where Ep-CAM was present at the lateral membrane including the adherens junction areas, but was fully excluded from the apical cell membrane (microvilli), tight junctions, and desmosomes. We conclude that (1) Ep-CAM does not form junctional complexes in L cells, (2) in epithelial cells, cell surface Ep-CAM is present at the lateral cell membrane, but is excluded from tight junctions and desmosomes, and (3) in epithelial cells, Ep-CAM is present within adhesions mediated by the classic cadherins (especially E-cadherin) with both types of molecules remaining as independent clusters. The colocalization with cadherins might be important for the modulating effect of Ep-CAM on cadherin-mediated adhesions.

CEA adhesion molecules: multifunctional proteins with signal-regulatory properties

The carcinoembryonic antigen family comprises a large number of complex molecules, several of which possess cell adhesion activities. The primordial adhesion molecules of this family are the cell-cell adhesion molecules (C-CAMs), which have been found to be multifunctional, signal-regulatory proteins. C-CAMs inhibit tumor growth, interact with calmodulin, protein tyrosine kinases and protein tyrosine phosphatases, and are subject to specific dimerization reactions. These new insights indicate that C-CAMs are important regulators of cellular functions.

Calmodulin binds to specific sequences in the cytoplasmic domain of C-CAM and down-regulates C-CAM self-association

C-CAM is a cell adhesion molecule belonging to the immunoglobulin supergene family and is known to mediate calcium-independent homophilic cell-cell binding. Two major isoforms, C-CAM1 and C-CAM2, which differ in their cytoplasmic domains, have been identified. Previous investigations have demonstrated that both cytoplasmic domains can bind calmodulin in a calcium-dependent reaction. In this investigation, peptides corresponding to the cytoplasmic domains of C-CAM were synthesized on cellulose membranes and used to map the binding sites for 125I-labeled calmodulin. Both C-CAM1 and C-CAM2 had one strong calmodulin-binding site in the membrane-proximal region. Those binding regions were conserved in C-CAM from rat, mouse, and man. In addition, C-CAM1 from rat and mouse contained a weaker binding site in the distal region of the cytoplasmic domain. Biosensor experiments were performed to determine rate and equilibrium constants of the C-CAM/calmodulin interaction. An association rate constants of 3.3 x 10(5) M-1 s-1 and two dissociation rate constants of 2.2 x 10(-2) and 3.1 x 10(-5) s-1 were determined. These correspond to equilibrium dissociation constants of 6.7 x 10(-8) and 9.4 x 10(-11) M, respectively. In dot-blot binding experiments, it was found that binding of calmodulin causes a down-regulation of the homophilic self-association of C-CAM. This suggests that calmodulin can regulate the functional activity of C-CAM.

Cell adhesion activity of the short cytoplasmic domain isoform of C-CAM (C-CAM2) in CHO cells

C-CAM is a Ca(2+)-independent rat cell adhesion molecule belonging to the CEA gene family of the immunoglobulin superfamily. Two major isoforms that differ in the length of their cytoplasmic domains exist. In previous studies it has been reported that only the long isoform (C-CAM1) but not the short isoform (C-CAM2) can mediate adhesion. However, in the mouse, isoforms with both long and short cytoplasmic domains have been reported to have adhesive activity. In order to analyze this apparent conflict we transfected C-CAM1 or C-CAM2 into CHO Pro5 cells and examined their adhesive phenotype in an aggregation assay. We found that in this cellular system both C-CAM1 and C-CAM2 could mediate cell-cell adhesion in a Ca(2+)-independent and temperature-independent way. The results suggest that the cellular environment is important for the activity of C-CAM isoforms.