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

CEA-Related CAMs

The carcinoembryonic antigen (CEA) family comprises a large number of cellular surface molecules, the CEA-related cell adhesion molecules (CEACAMs), which belong to the Ig superfamily. CEACAMs exhibit a complex expression pattern in normal and malignant tissues. The majority of the CEACAMs are cellular adhesion molecules that are involved in a great variety of distinct cellular processes, for example in the integration of cellular responses through homo- and heterophilic adhesion and interaction with a broad selection of signal regulatory proteins, i.e., integrins or cytoskeletal components and tyrosine kinases. Moreover, expression of CEACAMs affects tumor growth, angiogenesis, cellular differentiation, immune responses, and they serve as receptors for commensal and pathogenic microbes. Recently, new insights into CEACAM structure and function became available, providing further elucidation of their kaleidoscopic functions.

Regulation of endothelial connexin40 expression by shear stress

Endothelial connexin (Cx)40 plays an important role in signal propagation along blood vessel walls, modulating vessel diameter and thereby blood flow. Blood flow, in turn, has been shown to alter endothelial Cx40 expression. However, the timing and shear stress dependence of this relationship have remained unclear, as have the signal transduction pathways involved and the functional implications. Therefore, the aim of this study was to quantify the effects of shear stress on endothelial Cx40 expression, to analyze the role of phosphoinositide 3-kinase (PI3K)/Akt signaling involved, and to assess the possible functional consequences for the adaptation of microvascular networks. First-passage human umbilical vein endothelial cells were exposed to defined shear stress conditions and analyzed for Cx40 using real-time RT-PCR and immunoblot analysis. Shear stress caused long-term induction of Cx40 protein expression, with two short-term mRNA peaks at 4 and 16 h, indicating the dynamic nature of the adaptation process. Maximum shear stress-dependent induction was observed at shear levels between 6 and 10 dyn/cm(2). Simulation of this pattern of shear-dependent Cx expression in a vascular adaptation model of a microvascular network led to an improved fit for the simulated results to experimental measurements. Cx40 expression was greatly reduced by inhibiting PI3K or Akt, with PI3K activity being required for basal Cx40 expression and Akt activity taking part in its shear stress-dependent induction.

The CEACAM1 transmembrane domain, but not the cytoplasmic domain, directs internalization of human pathogens via membrane microdomains

Several bacterial pathogens exploit carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) to promote attachment and uptake into eukaryotic host cells. The widely expressed isoform CEACAM1 is involved in cell-cell adhesion, regulation of cell proliferation, insulin homeostasis, and neo-angiogenesis, processes that depend on the cytoplasmic domain of CEACAM1. By analysing the molecular requirements for CEACAM1-mediated internalization of bacteria, we surprisingly find that the CEACAM1 cytoplasmic domain is completely obsolete for bacterial uptake. Accordingly, CEACAM1-4L as well as a CEACAM1 mutant with a complete deletion of the cytoplasmic domain (CEACAM1 DeltaCT) promote equivalent internalization of several human pathogens. CEACAM1-4L- and CEACAM1 DeltaCT-mediated uptake proceeds in the presence of inhibitors of actin microfilament dynamics, which is in contrast to CEACAM3-mediated internalization. Bacteria-engaged CEACAM1-4L and CEACAM1 DeltaCT, but not CEACAM3, localize to a gangliosid GM1- and GPI-anchored protein-containing portion of the plasma membrane. In addition, interference with cholesterol-rich membrane microdomains severely blocks bacterial uptake via CEACAM1-4L and CEACAM1 DeltaCT, but not CEACAM3. Similar to GPI-anchored CEACAM6, both CEACAM1-4L as well as CEACAM1 DeltaCT partition into a low-density, Triton-insoluble membrane fraction upon receptor clustering, whereas CEACAM3 is not detected in this fraction. Bacterial uptake by truncated CEACAM1 or chimeric CEACAM1/CEACAM3 molecules reveals that the transmembrane domain of CEACAM1 is responsible for its association with membrane microdomains. Together, these data argue for a functional role of lipid rafts in CEACAM1-mediated endocytosis that is promoted by the transmembrane domain of the receptor and that might be relevant for CEACAM1 function in physiologic settings.

Regulation of Foxo-1 and the angiopoietin-2/Tie2 system by shear stress

Transcription factor Foxo-1 can be inactivated via Akt-mediated phosphorylation. Since shear stress activates Akt, we determined whether Foxo-1 and the Foxo-1-dependent, angiogenesis-related Ang-2/Tie2-system are influenced by shear stress in endothelial cells. Expression of Foxo-1 and its target genes p27Kip1 and Ang-2 was decreased under shear stress (6dyn/cm(2), 24h), nuclear exclusion of Foxo-1 by phosphorylation increased. eNOS and Tie2 were upregulated. No effects on Ang-1 expression were detected. In conclusion, Foxo-1 and Ang-2/Tie2 are part of the molecular response to shear stress, which may regulate angiogenesis.

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.

Vascular endothelial growth factor is expressed in endothelial cells isolated from skeletal muscles of nitric oxide synthase knockout mice during prazosin-induced angiogenesis

In skeletal muscles, angiogenesis can be induced by increases in wall shear stress. To identify molecules involved in the angiogenic process, a method based on the use of BS-1 lectin-coated magnetic beads was developed to isolate a cellular fraction enriched in microvascular endothelial cells which are directly exposed to wall shear stress. Using such cellular fractions from skeletal muscles of C57 mice in which angiogenesis was induced by administration with the alpha(1)-adrenergic antagonist prazosin, we found the concentration of vascular endothelial growth factor (VEGF) increased in correlation to the duration of the prazosin stimulus. In contrast, the angiopoietin-2/tie-2 system was not changed even after 4days of prazosin treatment. In neuronal nitric oxide synthase (nNOS) knockout mice, the VEGF concentration was also elevated after prazosin treatment but remained almost unchanged in endothelial nitric oxide synthase (eNOS) knockout mice. However, eNOS (and not nNOS) knockout mice expressed higher levels of VEGF under non-stimulated conditions as compared to C57 mice. These results suggest that VEGF produced in endothelial cells is involved in angiogenesis in skeletal muscles of mice responding to the administration of systemic vasodilators. NO derived from eNOS and nNOS may be an important regulator of the angiogenic response in skeletal muscles in vivo.

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.

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.

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

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