Avri Ben-Ze'ev

Autoregulation of E-cadherin expression by cadherin–cadherin interactions: the roles of β-catenin signaling, Slug, and MAPK

Transcriptional repression of E-cadherin, characteristic of epithelial to mesenchymal transition, is often found also during tumor cell invasion. At metastases, migratory fibroblasts sometimes revert to an epithelial phenotype, by a process involving regulation of the E-cadherin–β-catenin complex. We investigated the molecular basis of this regulation, using human colon cancer cells with aberrantly activated β-catenin signaling. Sparse cultures mimicked invasive tumor cells, displaying low levels of E-cadherin due to transcriptional repression of E-cadherin by Slug. Slug was induced by β-catenin signaling and, independently, by ERK. Dense cultures resembled a differentiated epithelium with high levels of E-cadherin and β-catenin in adherens junctions. In such cells, β-catenin signaling, ErbB-1/2 levels, and ERK activation were reduced and Slug was undetectable. Disruption of E-cadherin–mediated contacts resulted in nuclear localization and signaling by β-catenin, induction of Slug and inhibition of E-cadherin transcription, without changes in ErbB-1/2 and ERK activation. This autoregulation of E-cadherin by cell–cell adhesion involving Slug, β-catenin and ERK could be important in tumorigenesis.

Protein synthesis requires cell-surface contact while nuclear events respond to cell shape in anchorage-dependent fibroblasts

Anchorage-dependent mouse fibroblasts grow only if attached to and spread on a solid substrate. The suspension of cells in methyl-cellulose results in dramatic, coordinated inhibition of the major RNA and protein synthesis systems, and these systems are sequentially restored when cells are replated on a tissue culture dish surface. In the present report the effects on metabolism of cell reattachment are separated from those of subsequent spreading by controlling cell shape. Macromolecular metabolism is first strongly suppressed by long-term suspension culture. The cells are then replated in the presence of a variety of spreading inhibitors. The recovery of protein synthesis, which rapidly follows reattachment, does not require extensive cell spreading. Contact of a limited portion of the plasma membrane with the solid culture dish surface is apparently a sufficient signal by itself. A very different method of controlling cell shape is afforded by changing culture dish surface adhesivity. Suspended cells are replated on dishes precoated with thin layers of the hydrophilic hydrogen poly(2-hydroxyethyl methacrylate). The final mean cell diameter is then varied over wide limits. As before, protein synthesis recovery is unaffected. However, nuclear events such as DNA and rRNA synthesis and mRNA production are profoundly affected by cell shape. Thus, cell surface contact and cell shape give rise to distinctly different regulatory responses.

Differential molecular interactions of β-catenin and plakoglobin in adhesion, signaling and cancer

Plakoglobin and beta-catenin are homologous proteins functioning in cell adhesion and transactivation. Their activities are controlled by three types of interactions: those with cadherins in adherens junctions, linking them to the actin cytoskeleton; interactions in the nucleus, where they bind to transcription factors and stimulate gene expression; interactions of free cytoplasmic beta-catenin with axin and adenomatous polyposis coli (APC) protein which target it for degradation. Studies in the past year have demonstrated the complex interplay between these three types of interactions and the different behavior of beta-catenin and plakoglobin in their involvement in morphogenesis and tumorigenesis strongly suggesting that catenins play key roles in adhesion-mediated signaling.

Nuclear accumulation of p53 protein is mediated by several nuclear localization signals and plays a role in tumorigenesis.

The basic carboxy terminus of p53 plays an important role in directing the protein into the nuclear compartment. The C terminus of the p53 molecule contains a cluster of several nuclear localization signals (NLSs) that mediate the migration of the protein into the cell nucleus. NLSI, the most active domain, is highly conserved in genetically diverged species and shares perfect homology with consensus NLS sequences found in other nuclear proteins. The other two NLSs, II and III, appear to be less effective and less conserved. Although nuclear localization is dictated primarily by the NLSs inherent in the primary amino acid sequence, the actual nuclear homing can be modified by interactions with other proteins expressed in the cell. Comparison between wild-type p53 and naturally occurring mutant p53 showed that both protein categories could migrate into the nucleus of rat primary embryonic fibroblasts by essentially similar mechanisms. Nuclear localization of both proteins was totally dependent on the existence of functional NLS domains. In COS cells, however, we found that NLS-deprived wild-type p53 molecules could migrate into the nucleus by complexing with another nuclear protein, simian virus 40 large-T antigen. Wild-type and mutant p53 proteins differentially complexed with viral or cellular proteins, which may significantly affect the ultimate compartmentalization of p53 in the cell; this finding suggests that the actual subcellular compartmentalization of proteins may differ in various cell type milieux and may largely be affected by the ability of these proteins to complex with other proteins expressed in the cell. Experiments designed to test the physiological significance of p53 subcellular localization indicated that nuclear localization of mutant p53 is essential for this protein to enhance the process of malignant transformation of partially transformed cells, suggesting that p53 functions within the cell nucleus.

Mechanisms of regulating tubulin synthesis in cultured mammalian cells

Colchicine and nocadazole both depolymerize microtubules in cultured fibroblasts and lead to a rapid inhibition of tubulin synthesis. The level of translatable tubulin mRNA is greatly reduced in drug-treated cells as demonstrated by translation in a reticulocyte-derived in vitro protein synthesizing system. A model of tubulin synthesis regulation is proposed in which the elevated level of unpolymerized tubulin in drug-treated cells inhibits the formation of new tubulin mRNA and the preexisting message decays rapidly. In agreement with this model, tubulin message is found to be short-lived and has an approximately 2 hr half-life in cells treated with actinomycin D. Another prediction of the proposed model is that destabilization of microtubules without a concomitant increase in free tubulin will not inhibit tubulin synthesis. Vinblastine also disrupts microtubules but leads to the aggregation of tubulin into large paracrystals with an apparent decrease in the concentration of free tubulin. This drug does not inhibit tubulin production but rather leads to a measurable enhancement of tubulin synthesis.

The control of mRNA production, translation and turnover in suspended and reattached anchorage-dependent fibroblasts

Abstract Strictly anchorage-dependent 3T6 fibroblasts require contact with a solid surface for growth and for normal macromolecular metabolism. Denial of surface contact or anchorage by suspension in methocel leads to dramatic changes in messenger RNA metabolism and protein synthesis. These are reversed when cells contact a surface, even when spreading is prevented by agents such as concanavalin A. The responses to suspension appear to be well ordered regulatory events. While the synthesis of hnRNA remains constant, the production of mRNA is reduced 5 fold within a few hours under suspension conditions, suggesting a possible post-transcriptional control mechanism. Normally turning over messenger RNA is stabilized; the breakdown rate is reduced to match the lowered rate of message production so that the total amount of mRNA in the suspended fibroblasts remains constant. Protein synthesis declines slowly but extensively, and amounts to only 15% of control cells after 72 hr of suspension. The rate of protein synthesis is clearly not regulated by the amount of mRNA present, but rather by its availability. This is shown by the 6–7 fold increase of protein synthesis which occurs within 4 hr after the reattachment of suspended cells to a solid substrate. Messenger RNA production remains low during this period and cannot account for the resumption of protein synthesis. Rather, the recovery utilizes preexisting mRNA that has been translationally inactive, and this mRNA, once returned to translation, is engaged on full-sized polyribosomes, indicating the absence of a lesion in polypeptide initiation. At least one major protein, of about 43,000 dalton molecular weight, shows greatly enhanced synthesis during recovery. This polypeptide comigrates with actin in two-dimensional gel electrophoresis and is extensively associated with the cytoskeleton. Its enhanced synthesis following cell reattachment may represent a modulation of a morphology-related gene.

The outer boundary of the cytoskeleton: a lamina derived from plasma membrane proteins

We prepared the cytoskeletal framework by gently extracting cells with Triton X-100. Lipids and soluble proteins were removed, leaving a complex meshlike structure which contains the cell nucleus and is composed of the major cell filament networks as well as the microtrabeculae with attached polyribosomes. The surface sheet or lamina covering this structure contains most of the cell surface proteins by the following criteria. Intact cells are labeled externally with radioiodine and then extracted with detergent. The iodinated poteins remain almost entirely with skeletal framework. A new major integral protein, the coat protein of Sindbis virus, is inserted into the plasma membrane of infected cells. This new protein is heavily iodinated and remains almost completely associated with the framework after extraction. Lectin binding and poliovirus binding sites are also retained after detergent extraction. Our results indicate that plasma membrane proteins form a sheet or lamina upon removal of lipids. This lamina reproduces even complex surface convolutions and appears to be supported by and intimately connected to the underlying skeleton. In this case, the surface lamina, and hence the plasma membrane of the original intact cell, might be viewed as a component of the cytoskeletal framework.

Excess beta-catenin promotes accumulation of transcriptionally active p53.

β‐catenin is a multifunctional protein, acting both as a structural component of the cell adhesion machinery and as a transducer of extracellular signals. Deregulated β‐catenin protein expression, due to mutations in the β‐catenin gene itself or in its upstream regulator, the adenomatous polyposis coli (APC) gene, is prevalent in colorectal cancer and in several other tumor types, and attests to the potential oncogenic activity of this protein. Increased expression of β‐catenin is an early event in colorectal carcinogenesis, and is usually followed by a later mutational inactivation of the p53 tumor suppressor. To examine whether these two key steps in carcinogenesis are interrelated, we studied the effect of excess β‐catenin on p53. We report here that overexpression of β‐catenin results in accumulation of p53, apparently through interference with its proteolytic degradation. This effect involves both Mdm2‐dependent and ‐independent p53 degradation pathways, and is accompanied by augmented transcriptional activity of p53 in the affected cells. Increased p53 activity may provide a safeguard against oncogenic deregulation of β‐catenin, and thus impose a pressure for mutational inactivation of p53 during the later stages of tumor progression.

Fascin, a Novel Target of β-Catenin-TCF Signaling, Is Expressed at the Invasive Front of Human Colon Cancer

Cancer cells become metastatic by acquiring a motile and invasive phenotype. This step requires remodeling of the actin cytoskeleton and the expression of exploratory, sensory organelles known as filopodia. Aberrant beta-catenin-TCF target gene activation plays a major role in colorectal cancer development. We identified fascin1, a key component of filopodia, as a target of beta-catenin-TCF signaling in colorectal cancer cells. Fascin1 mRNA and protein expression were increased in primary cancers in a stage-dependent manner. Fascin1 was exclusively localized at the invasive front of tumors also displaying nuclear beta-catenin. Forced expression of fascin1 in colorectal cancer cells increased their migration and invasion in cell cultures and caused cell dissemination and metastasis in vivo, whereas suppression of fascin1 expression by small interfering RNA reduces cell invasion. Although expression of fascin1 in primary tumors correlated with the presence of metastases, fascin1 was not expressed in metastases. Our studies show that fascin1 expression is tightly regulated during development of colon cancer metastases and is a novel target of beta-catenin-TCF signaling. We propose that transient up-regulation of fascin1 in colorectal cancer promotes the acquisition of migratory and invasive phenotypes that lead to metastasis. Moreover, the expression of fascin1 is down-regulated when tumor cells reach their metastatic destination where migration ceases and proliferation is enhanced. Although metastasis to vital organs is often the cause of mortality, only limited success has been attained in developing effective therapeutics against metastatic disease. We propose that genes involved in cell migration and invasion, such as fascin1, could serve as novel targets for metastasis prevention.

Modulation of bacterial entry into epithelial cells by association between vinculin and the Shigella IpaA invasin

Shigella flexneri is the causative agent of bacillary dysentery in humans. Shigella invasion of epithelial cells is characterized by cytoskeletal rearrangements and formation of cellular projections engulfing the bacterium in a macropinocytic process. We show here that vinculin, a protein involved in linking actin filaments to the plasma membrane, is a direct target of Shigella during cell invasion. IpaA, a Shigella protein secreted upon cell contact, rapidly associates with vinculin during bacterial invasion. Although defective for cell entry, an ipaA mutant is still able to induce foci of actin polymerization, but differs from wild‐type Shigella in its ability to recruit vinculin and α‐actinin. Presumably, IpaA–vinculin interaction initiates the formation of focal adhesion‐like structures required for efficient invasion.