Anti-peptide Sera Against Cell-CAM 105 Determine High Molecular-mass Variants of the Long Isoform in Rat Hepatocytes

The transmembrane domain of CEACAM1-4S is a determinant of anchorage independent growth and tumorigenicity

CEACAM1 is a multifunctional Ig-like cell adhesion molecule expressed by epithelial cells in many organs. CEACAM1-4L and CEACAM1-4S, two isoforms produced by differential splicing, are predominant in rat liver. Previous work has shown that downregulation of both isoforms occurs in rat hepatocellular carcinomas. Here, we have isolated an anchorage dependent clone, designated 253T-NT that does not express detectable levels of CEACAM1. Stable transfection of 253-NT cells with a wild type CEACAM1-4S expression vector induced an anchorage independent growth in vitro and a tumorigenic phenotype in vivo. These phenotypes were used as quantifiable end points to examine the functionality of the CEACAM1-4S transmembrane domain. Examination of the CEACAM1 transmembrane domain showed N-terminal GXXXG dimerization sequences and C-terminal tyrosine residues shown in related studies to stabilize transmembrane domain helix-helix interactions. To examine the effects of transmembrane domain mutations, 253-NT cells were transfected with transmembrane domain mutants carrying glycine to leucine or tyrosine to valine substitutions. Results showed that mutation of transmembrane tyrosine residues greatly enhanced growth in vitro and in vivo. Mutation of transmembrane dimerization motifs, in contrast, significantly reduced anchorage independent growth and tumorigenicity. 253-NT cells expressing CEACAM1-4S with both glycine to leucine and tyrosine to valine mutations displayed the growth-enhanced phenotype of tyrosine mutants. The dramatic effect of transmembrane domain mutations constitutes strong evidence that the transmembrane domain is an important determinant of CEACAM1-4S functionality and most likely by other proteins with transmembrane domains containing dimerization sequences and/or C-terminal tyrosine residues.

Identification of a novel protein, LYRIC, localized to tight junctions of polarized epithelial cells

Tight junctions (TJ) are multiprotein complexes that function to regulate paracellular transport of molecules through epithelial and endothelial cell layers. Many new tight junction-associated proteins have been identified in the past few years, and their functional roles and interactions have just begun to be elucidated. In this paper, we describe a novel protein LYsine-RIch CEACAM1 co-isolated (LYRIC) that is widely expressed and highly conserved between species. LYRIC has no conserved domains that would indicate function and does not appear to be a member of a larger protein family. Data from analysis of rat and human tissue sections and cell lines show that LYRIC colocalizes with tight junction proteins ZO-1 and occludin in polarized epithelial cells, suggesting that LYRIC is part of the tight junction complex. LYRIC dissociates from ZO-1 when junctional complexes are disrupted, and as tight junctions reform, ZO-1 relocalizes before LYRIC. These results suggest that LYRIC is most likely not a structural component required for TJ formation, but rather is recruited during the maturation of the tight junction complex.

Two Variable Regions in Carcinoembryonic Antigen-related Cell Adhesion Molecule1 N-terminal Domains Located in or Next to Monoclonal Antibody and Adhesion Epitopes Show Evidence of Recombination in Rat but Not in Human

In this paper, we have characterized the structure, evolutionary origin, and function of rat and human carcinoembryonic antigen-related cell adhesion molecule1 (CEACAM1) multifunctional Ig-like cell adhesion proteins that are expressed by many epithelial tissues. Restriction enzyme digestion reverse transcriptase-PCR analysis identified three cDNAs encoding novel CEACAM1 N-domains. Comparative sequence analysis showed that human and rat CEACAM1 N-domains segregated into two groups differing in similarity to rat CEACAM1(a)-4L and human CEACAM1. Sequence variability analysis indicated that both human and rat N-domains possessed two variable regions, and one contained a major adhesive epitope. Recombination analysis showed that the group of rat but not human N-domains with high sequence similarity was derived at least in part by recombination. Binding assays revealed that three monoclonal antibodies with strong reactivity for the CEACAM1(a)-4L N-domain showed no reactivity with CEACAM1(b)-4S, an allele with a different N-domain sequence. CEACAM1(b)-4S displayed adhesive activity efficiently blocked by a synthetic peptide corresponding to the adhesive epitope in CEACAM1(a)-4L. Blocking analysis also showed that the adhesive epitope for rat CEACAM1 was located downstream from the equivalent human and mouse epitopes. Glycosylation analysis demonstrated O-linked sugars on rat CEACAM1(b)-4S from COS-1 cells. However, this was not the alteration responsible for the lack of monoclonal antibody reactivity. When considered together with previous studies, our findings suggest an inverse relationship between functionality and amino acid sequence similarity to CEACAM1. Like IgG, the N-domain of CEACAM1 appears to tolerate 10-15% sequence diversification without loss of function but begins to show either altered specificity or diminished functionality at higher levels.

Clustering-induced signaling of CEACAM1 in PC12 cells

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), an Ig-like transmembrane protein, functions in cell adhesion, angiogenesis and epithelial cell morphogenesis, and has been identified as a tumor suppressor. For all of these functions, CEACAM1 requires signaling capabilities. However, the mechanisms of CEACAM1-mediated signaling are only poorly understood. Here we characterized for the first time CEACAM1 expression and signaling in the neuroendocrine rat pheochromocytoma PC12 cell line. Stimulation of CEACAM1 by ligation on the cell surface with antibodies induced formation of large CEACAM1 clusters and a rapid and transient CEACAM1 tyrosine dephosphorylation. Functionally, this dephosphorylation correlated with a reduced association between CEACAM1 and the tyrosine phosphatase SHP2. Clustering also stimulated binding of CEACAM1 to the actin cytoskeleton, measured by a partial translocation of CEACAM1 into the insoluble fraction after detergent extraction. Both tyrosine dephosphorylation and interaction with the cytoskeleton were sensitive to neuronal differentiation of PC12 cells. The first detected downstream activation of the mitogen-activated protein kinases ERK1 and ERK2, but not of JNK or p38, describes a novel target of CEACAM1-mediated signaling and contributes to the understanding of how CEACAM1 regulates cellular function.

The long isoform of the cell adhesion molecule C-CAM binds to actin

C-CAM is a member of the carcinoembryonic antigen family (CEA) of the rat, which mediates cell adhesion in vitro and binds to signal transduction molecules. In many tissues C-CAM is expressed in the apical domain of the plasma membrane in close contact with intracellular cortical microfilaments, e.g., in the microvilli of the brush borders of enterocytes. Regarding this subcellular localisation, we have investigated the C-CAM interaction with the cytoskeleton. The association of C-CAM with detergent-insoluble structures increased when the small intestinal mucosa was extracted under conditions known to preserve the cytoskeleton of the brush borders. We found a co-immunoprecipitation of actin with C-CAM of the small intestine mucosa which increased in the presence of the chemical cross-linker DSP, allowing the demonstration of complexes between C-CAM and actin of different molecular masses. A recombinant fusion protein of the cytoplasmic domain of the long isoform of C-CAM bound specifically to purified actin in a co-sedimentation assay. These results suggest an intrinsic actin-binding activity of C-CAM.

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

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

Characterization of molecular aggregates of alpha 1 beta 1-integrin and other rat liver membrane proteins by combination of size-exclusion chromatography and chemical cross-linking

Many membrane proteins display their biological activity in molecular aggregates of interacting counterparts. The analysis of these aggregates remains difficult; especially intermolecular complexes of membrane proteins tend to dissociate or artificially aggregate during detergent extraction out of membranes. Thus, the existence of protein aggregates was investigated by two approaches. First, after modest detergent extraction, the presence of three well characterized rat liver membrane proteins, alpha 1 beta 1-integrin, dipeptidyl aminopeptidase IV (DPP IV) and cell-CAM 105 (CAM = cell adhesion molecule), in aggregates could be demonstrated when investigated by size-exclusion chromatography (SEC) under non-denaturating conditions. However, the applied detergents partially influenced the resolution of the separation reducing the ability to discriminate between native and artificial protein aggregates. To circumvent these problems, a second approach based on covalent cross-linking of native protein complexes by dithiobis(succinimidylpropionate) was combined with the performance of denaturating SEC. Under such optimized some high-molecular-mass complexes of all model proteins consisting of unknown components could also be detected. Taken together, non-denaturating SEC and chemical cross-linking in combination with denaturating SEC represent methodological approaches for the characterization of protein aggregates.