Retraction: miR-661 expression in SNAI1-induced epithelial to mesenchymal transition contributes to breast cancer cell invasion by targeting Nectin-1 and StarD10 messengers.

At the request of the University of Luxembourg and following an external investigation, the Editor and Publisher have agreed to retract this paper owing to unreliable data.

ETV5 cooperates with LPP as a sensor of extracellular signals and promotes EMT in endometrial carcinomas.

Endometrial carcinoma (EC) is the most frequent among infiltrating tumors of the female genital tract, with myometrial invasion representing an increase in the rate of recurrences and a decrease in survival. We have previously described ETV5 transcription factor associated with myometrial infiltration in human ECs. In this work, we further investigated ETV5 orchestrating downstream effects to confer the tumor the invasive capabilities needed to disseminate in the early stages of EC dissemination. Molecular profiling evidenced ETV5 having a direct role on epithelial-to-mesenchymal transition (EMT). In particular, ETV5 modulated Zeb1 expression and E-Cadherin repression leading to a complete reorganization of cell-cell and cell-substrate contacts. ETV5-promoted EMT resulted in the acquisition of migratory and invasive capabilities in endometrial cell lines. Furthermore, we identified the lipoma-preferred partner protein as a regulatory partner of ETV5, acting as a sensor for extracellular signals promoting tumor invasion. All together, we propose ETV5-transcriptional regulation of the EMT process through a crosstalk with the tumor surrounding microenvironment, as a principal event initiating EC invasion.

miR-661 expression in SNAI1-induced epithelial to mesenchymal transition contributes to breast cancer cell invasion by targeting Nectin-1 and StarD10 messengers.

Epithelial to mesenchymal transition (EMT) is a key step toward metastasis. MCF7 breast cancer cells conditionally expressing the EMT master regulator SNAI1 were used to identify early expressed microRNAs (miRNAs) and their targets that may contribute to the EMT process. Potential targets of miRNAs were identified by matching lists of in silico predicted targets and of inversely expressed mRNAs. MiRNAs were ranked based on the number of predicted hits, highlighting miR-661, a miRNA with so far no reported role in EMT. MiR-661 was found required for efficient invasion of breast cancer cells by destabilizing two of its predicted mRNA targets, the cell-cell adhesion protein Nectin-1 and the lipid transferase StarD10, resulting, in turn, in the downregulation of epithelial markers. Reexpression of Nectin-1 or StarD10 lacking the 3'-untranslated region counteracted SNAI1-induced invasion. Importantly, analysis of public transcriptomic data from a cohort of 295 well-characterized breast tumor specimen revealed that expression of StarD10 is highly associated with markers of luminal subtypes whereas its loss negatively correlated with the EMT-related, basal-like subtype. Collectively, our non-a priori approach revealed a nonpredicted link between SNAI1-triggered EMT and the down-regulation of Nectin-1 and StarD10 through the up-regulation of miR-661, which may contribute to the invasion of breast cancer cells and poor disease outcome.

Time-resolved analysis of transcriptional events during SNAI1-triggered epithelial to mesenchymal transition.

The transcription regulator SNAI1 triggers a transcriptional program leading to epithelial to mesenchymal transition (EMT), providing epithelial cells with mesenchymal features and invasive properties during embryonic development and tumor progression. To identify early transcriptional changes occurring during SNAI1-induced EMT, we performed a time-resolved genome-scale study using human breast carcinoma cells conditionally expressing SNAI1. The approach we developed for microarray data analysis, allowed identifying three distinct EMT stages and the temporal classification of genes. Importantly, we identified unexpected, biphasic expression profiles of EMT-associated genes, supporting their pivotal role during this process. Finally, we established early EMT gene networks by identifying transcription factors and their potential targets which may orchestrate early events of EMT. Collectively, our work provides a framework for the identification and future systematic analysis of novel genes which contribute to SNAI1-triggered EMT.

ActA and human zyxin harbour Arp2/3-independent actin-polymerization activity.

The actin cytoskeleton is a dynamic network that is composed of a variety of F-actin structures. To understand how these structures are produced, we tested the capacity of proteins to direct actin polymerization in a bead assay in vitro and in a mitochondrial-targeting assay in cells. We found that human zyxin and the related protein ActA of Listeria monocytogenes can generate new actin structures in a vasodilator-stimulated phosphoprotein-dependent (VASP) manner, but independently of the Arp2/3 complex. These results are consistent with the concept that there are multiple actin-polymerization machines in cells. With these simple tests it is possible to probe the specific function of proteins or identify novel molecules that act upon cellular actin polymerization.

Targeting of zyxin to sites of actin membrane interaction and to the nucleus.

The localization of proteins to particular intracellular compartments often regulates their functions. Zyxin is a LIM protein found prominently at sites of cell adhesion, faintly in leading lamellipodia, and transiently in cell nuclei. Here we have performed a domain analysis to identify regions in zyxin that are responsible for targeting it to different subcellular locations. The N-terminal proline-rich region of zyxin, which harbors binding sites for alpha-actinin and members of the Ena/VASP family, concentrates in lamellipodial extensions and weakly in focal adhesions. The LIM region of zyxin displays robust targeting to focal adhesions. When overexpressed in cells, the LIM region of zyxin causes displacement of endogenous zyxin from focal adhesions. Upon mislocalization of full-length zyxin, at least one member of the Ena/VASP family is also displaced, and the organization of the actin cytoskeleton is perturbed. Zyxin also has the capacity to shuttle between the nucleus and focal adhesion sites. When nuclear export is inhibited, zyxin accumulates in cell nuclei. The nuclear accumulation of zyxin occurs asynchronously with approximately half of the cells exhibiting nuclear localization of zyxin within 2.3 h of initiating leptomycin B treatment. Our results provide insight into the functions of different zyxin domains.

Expression patterns of L-plastin isoform in normal and carcinomatous breast tissues.

Plastins are members of a family of actin-binding proteins which exhibit a tissue-specific expression pattern. L-plastin, which is specifically expressed in hematopoietic cell lineage, has been proposed to be involved in the control of cell adhesion and motility. This protein is also frequently expressed in cell lines derived from mammary solid tumors and therefore might be involved in cancer invasion and metastasis. We have analysed plastin expression in normal and carcinomatous breast tissues in vivo by immunohistochemistry and immunoblotting approaches using specific plastin isoform antibodies. L-plastin was not detected in normal epithelial cells of the mammary gland whereas a staining of myoepithelial cells was observed in 50% of the cases. In breast carcinomas, a significant immunostaining of malignant epithelial cells was observed in 4 of the 29 cases analysed (13.8%). No correlation between L-plastin expression and tumor size, histological grade or lymph node status was observed. In contrast, L-plastin was found expressed in 4 of the 11 estrogen and progesterone receptors negative tumors (p = 0.039). The potential role of plastin expression in the tumor process is discussed.

Characterization of the interaction between zyxin and members of the Ena/vasodilator-stimulated phosphoprotein family of proteins.

Zyxin contains a proline-rich N-terminal domain that is similar to the C-terminal domain in the ActA protein of the bacteria, Listeria monocytogenes. We screened the entire amino acid sequence of human zyxin for Mena-interacting peptides and found that, as with ActA, proline-rich sequences were the sole zyxin sequences capable of binding to Ena/vasodilator-stimulated phosphoprotein (VASP) family members in vitro. From this information, we tested zyxin mutants in which the proline-rich sequences were altered. The reduction in Mena/VASP binding was confirmed by peptide tests, immunoprecipitation, and ectopic expression of zyxin variants at the surface of mitochondria. By transfection assays we showed that zyxin interaction with Mena/VASP in vivo enhances the production of actin-rich structures at the apical surface of cells. Microinjection into cells of peptides corresponding to the first proline-rich sequence of zyxin caused the loss of Mena/VASP from focal contacts. Furthermore, these peptides reduced the degree of spreading of cells replated after trypsinization. We conclude that zyxin and proteins that harbor similar proline-rich repeats contribute to the positioning of Mena/VASP proteins. The positioning of Ena/VASP family members appears to be important when the actin cytoskeleton is reorganized, such as during spreading.

Growing an actin gel on spherical surfaces.

Inspired by the motility of the bacteria Listeria monocytogenes, we have experimentally studied the growth of an actin gel around spherical beads grafted with ActA, a protein known to be the promoter of bacteria movement. On ActA-grafted beads F-actin is formed in a spherical manner, whereas on the bacteria a "comet-like" tail of F-actin is produced. We show experimentally that the stationary thickness of the gel depends on the radius of the beads. Moreover, the actin gel is not formed if the ActA surface density is too low. To interpret our results, we propose a theoretical model to explain how the mechanical stress (due to spherical geometry) limits the growth of the actin gel. Our model also takes into account treadmilling of actin. We deduce from our work that the force exerted by the actin gel on the bacteria is of the order of 10 pN. Finally, we estimate from our theoretical model possible conditions for developing actin comet tails.

Villin-like actin-binding proteins are expressed ubiquitously in Arabidopsis.

In an attempt to elucidate the biological function of villin-like actin-binding proteins in plants we have cloned several genes encoding Arabidopsis proteins with high homology to animal villin. We found that Arabidopsis contains at least four villin-like genes (AtVLNs) encoding four different VLN isoforms. Two AtVLN isoforms are more closely related to mammalian villin in their primary structure and are also antigenically related, whereas the other two contain significant changes in the C-terminal headpiece domain. RNA and promoter/beta-glucuronidase expression studies demonstrated that AtVLN genes are expressed in all organs, with elevated expression levels in certain types of cells. These results suggest that AtVLNs have less-specialized functions than mammalian villin, which is found only in the microvilli of brush border cells. Immunoblot experiments using a monoclonal antibody against pig villin showed that AtVLNs are widely distributed in a variety of plant tissues. Green fluorescent protein fused to full-length AtVLN and individual AtVLN headpiece domains can bind to both animal and plant actin filaments in vivo.

LPP, an actin cytoskeleton protein related to zyxin, harbors a nuclear export signal and transcriptional activation capacity.

The LPP gene is the preferred translocation partner of the HMGIC gene in a subclass of human benign mesenchymal tumors known as lipomas. Here we have characterized the LPP gene product that shares 41% of sequence identity with the focal adhesion protein zyxin. LPP localizes in focal adhesions as well as in cell-to-cell contacts, and it binds VASP, a protein implicated in the control of actin organization. In addition, LPP accumulates in the nucleus of cells upon treatment with leptomycin B, an inhibitor of the export factor CRM1. The nuclear export of LPP depends on an N-terminally located leucine-rich sequence that shares sequence homology with well-defined nuclear export signals. Moreover, LPP displays transcriptional activation capacity, as measured by GAL4-based assays. Altogether, these results show that the LPP protein has multifunctional domains and may serve as a scaffold upon which distinct protein complexes are assembled in the cytoplasm and in the nucleus.

Villin function in the organization of the actin cytoskeleton. Correlation of in vivo effects to its biochemical activities in vitro.

Villin is an actin-binding protein of the intestinal brush border that bundles, nucleates, caps, and severs actin in a Ca(2+)-dependent manner in vitro. Villin induces the growth of microvilli in transfected cells, an activity that requires a carboxyl-terminally located KKEK motif. By combining cell transfection and biochemical assays, we show that the capacity of villin to induce growth of microvilli in cells correlates with its ability to bundle F-actin in vitro but not with its nucleating activity. In agreement with its importance for microfilament bundling in cells, the KKEK motif of the carboxyl-terminal F-actin-binding site is crucial for bundling in vitro. In addition, substitutions of basic residues in a second site, located in the amino-terminal portion of villin, impaired its activity in cells and reduced its binding to F-actin in the absence of Ca(2+) as well as its bundling and severing activities in vitro. Altogether, these findings suggest that villin participates in the organization and stabilization of the brush border core bundle but does not initiate its assembly by nucleation of actin filaments.

Quantitative measurement of proteins by western blotting with Cy5-coupled secondary antibodies.

The concentration of proteins in cells is an important parameter that determines how a protein will interact with other proteins or pharmacological agents. Recent developments in Western blotting techniques have now made this a method of choice to measure protein concentration in complex solutions such as total cell extracts. We show that detection of Cy5-coupled secondary antibodies by PhosphorImager analysis produces signals that approach linearity with respect to protein concentration over a 20-fold range. We used this technique to estimate cellular levels of zyxin, which is an important protein component of the actin cytoskeleton in mammalian cells. By producing specific protein standards based on sequences that are available from public databases, it is now possible to estimate the concentration of almost any protein by this technique.

Structural and functional similarities between the human cytoskeletal protein zyxin and the ActA protein of Listeria monocytogenes.

The intracellular bacterial parasite Listeria monocytogenes produces ActA protein at its surface to facilitate the localized assembly of actin-filled comets that are required for movement. The organization of actin in Listeria comets shows striking similarity to the organization of actin at the plasma membrane of mammalian cells. Therefore we examined the possibility that an ActA-like protein is present in mammalian cells. By using antibodies directed against ActA, we identified zyxin as an ActA related protein in a number of cell types. We compared the functions of ActA and zyxin by transient expression of variants tagged with an inner plasma membrane localization sequence (a CAAX box). Targeting of the proline rich domain of zyxin to the plasma membrane disrupts the actin cytoskeleton and cell shape in a manner similar to that which occurs with membrane-targeted ActA sequences. A chimeric protein composed of the N-terminal domain of ActA fused to the N-terminal and central domains of zyxin induced a full ActA response in cells. Furthermore, zyxin and ActA exhibit common protein partners in vitro. On the basis of the shared properties of zyxin and ActA, we propose that zyxin enhances actin organizing activity in mammalian cells.

The role of actin binding proteins in epithelial morphogenesis: models based upon Listeria movement.

We summarize recent findings on the organization of the protein actin in eucaryotic cells. In particular we focus on how actin can be used to generate a vectorial force that is required for cell movement. These forces arise from protein molecules that recruit actin to the plasma membrane in such a manner that actin filaments extend outward from the cell body. This type of actin dependent force generation has been described in a nucleation-release model, which is one of several models currently being tested to explain actin dependent cell movement. Data in support of this model has arisen unexpectedly from studies of an intracellular bacteria, Listeria monocytogenes. This bacteria uses actin to propel itself during infection of eucaryotic cells. By studying Listeria movement, the roles of several eucaryotic actin interacting proteins have been identified. One of these is zyxin, a human protein that shares important structural and possibly functional properties with ActA, an actin dependent force generating protein of Listeria. We intend to test the function of these and other actin interacting proteins in a simplified system that should facilitate precise measurement of their properties of force generation in vitro.

Conformational behaviour of a synthetic peptide of the C-terminus of villin that interacts with actin: an NMR, CD and stimulated annealing study.

The solution structure of a synthetic 22-amino acid peptide (P1) corresponding to the extreme C-terminal end and one of the F-actin binding sites of villin has been determined by 1H NMR and CD spectroscopy. The structure of this peptide was compared to that of a peptide in which lysine to glutamic acid substitutions were introduced at positions 17 and 19 (P11), abolishing F-actin binding. Both peptides are largely unstructured in aqueous solution. Changes observed in the NMR and CD spectra of both peptides are consistent with alpha-helix formation in trifluoroethanol/water mixtures. A set of 189 interproton distances derived from nuclear Overhauser enhancement (NOE) measurements, 17 phi-angle constraints obtained from 3JNH alpha coupling constants, as well as about 10 N ... O distance restraints deduced from amide proton exchange kinetics with deuterium, were used for the structure determination. The three-dimensional structure of P1 and P11 is characterized by two helical regions, one extending from residues 2 to 5 and a second covering residues 7 to 17. The central fragment, ranging from Leu-7 to Leu-15, is more stable. The C-terminal residues are less structured, particularly within peptide P11. The significance of these structural results is discussed in relation to the biological activity of villin.

Targeting of Listeria monocytogenes ActA protein to the plasma membrane as a tool to dissect both actin-based cell morphogenesis and ActA function.

Actin assembly on the surface of Listeria monocytogenes in the cytoplasm of infected cells provides a model to study actin-based motility and changes in cell shape. We have shown previously that the ActA protein, exposed on the bacterial surface, is required for polarized nucleation of actin filaments. To investigate whether plasma membrane-associated ActA can control the organization of microfilaments and cell shape, variants of ActA, in which the bacterial membrane signal had been replaced by a plasma membrane anchor sequence, were produced in mammalian cells. While both cytoplasmic and membrane-bound forms of ActA increased the F-actin content, only membrane-associated ActA caused the formation of plasma membrane extensions. This finding suggests that ActA acts as an actin filament nucleator and shows that permanent association with the inner face of the plasma membrane is required for changes in cell shape. Based on the observation that the amino-terminal segment of ActA and the remaining portion which includes the proline-rich repeats cause distinct phenotypic modifications in transfected cells, we propose a model in which two functional domains of ActA cooperate in the nucleation and dynamic turnover of actin filaments. The present approach is a new model system to dissect the mechanism of action of ActA and to further investigate interactions of the plasma membrane and the actin cytoskeleton during dynamic changes of cell shape.

Functional differences between L- and T-plastin isoforms.

Fimbrins/plastins are a family of highly conserved actin-bundling proteins. They are present in all eukaryotic cells including yeast, but each isoform displays a remarkable tissue specificity. T-plastin is normally found in epithelial and mesenchymal cells while L-plastin is present in hematopoietic cells. However, L-plastin has been also found in tumor cells of non-hematopoietic origin (Lin, C.-S., R. H. Aebersold, S. B. Kent, M. Varma, and J. Leavitt. 1988. Mol. Cell. Biol. 8:4659-4668; Lin, C.-S., R. H. Aebersold, and J. Leavitt. 1990. Mol. Cell. Biol. 10: 1818-1821). To learn more about the biological significance of their tissue specificity, we have overproduced the T- and L-plastin isoforms in a fibroblast-like cell line, CV-1, and in a polarized epithelial cell line, LLC-PK1. In CV-1 cells, overproduction of T- and L-plastins induces cell rounding and a concomitant reorganization of actin stress fibers into geodesic structures. L-plastin remains associated with microfilaments while T-plastin is almost completely extracted after treatment of the cells with non-ionic detergent. In LLC-PK1 cells, T-plastin induces shape changes in microvilli and remains associated with microvillar actin filaments after detergent extraction while L-plastin has no effect on these structures and is completely extracted. The effect of T-plastin on the organization of microvilli differs from that of villin, another actin-bundling protein. Our experiments indicate that these two isoforms play differing roles in actin filament organization, and do so in a cell type-specific fashion. Thus it is likely that these plastin isoforms play fundamentally different roles in cell function.

Villin-induced growth of microvilli is reversibly inhibited by cytochalasin D.

Villin is an actin-binding protein that is associated with the cytoskeleton of brush border microvilli. In vitro, villin nucleates, caps or severs actin filaments in a Ca(2+)-dependent manner. In the absence of Ca2+, villin organizes microfilaments into bundles. Transfection of a villin-specific cDNA into cultured cells that do not produce this protein results in the growth of long surface microvilli and the reorganization of the underlying actin cytoskeleton. Here we studied the effects of low concentrations of cytochalasin D on the induction of these plasma membrane-actin cytoskeleton specializations. Transfected cells were treated with concentrations of cytochalasin D that prevent the association of actin monomers with the fast-growing end of microfilaments in vitro. In villin-positive cells, cytochalasin D inhibited the growth of microvilli and promoted the formation of rodlet-like actin structures, which were randomly distributed throughout the cytoplasm. The formation of these structures was dependent on large amounts of villin and on the integrity of an actin-binding site located at the carboxy terminus of villin, which is required for microfilament bundling in vitro and for the growth of microvilli in vivo. The effect of cytochalasin D was reversible. The observation of living cells by video-imaging revealed that when cytochalasin D was removed, rapid disassembly of actin rodlets occurred after a lag phase. The present data stress the important role of the plasma membrane in the organization of the actin cytoskeleton and suggest that the extension of the microvillar plasma membrane is dependent on the elongation of microfilaments at their fast-growing end. Inhibition of microfilament elongation near the plasma membrane by cytochalasin D may result in the 'random' nucleation of actin filaments throughout the cytoplasm. On the basis of the present data, we propose that villin is involved in the assembly of the microvillar actin bundle by a mechanism that does not prevent monomer association with the preferred end of microfilaments. For instance, villin may stabilize actin filaments by lateral interactions. The functional importance of the carboxy-terminal F-actin binding site in such a mechanism is stressed by the fact that it is required for the formation of F-actin rodlets in cytochalasin D-treated cells. Finally, our data further emphasize the observations that the effects of cytochalasin D in living cells can be modulated by actin-binding proteins.

An actin-binding site containing a conserved motif of charged amino acid residues is essential for the morphogenic effect of villin.

The actin-binding protein villin induces microvillus growth and reorganization of the cytoskeleton in cells that do not normally produce this protein. Transfection of mutagenized villin cDNAs into CV-1 cells was used to show that a conserved, COOH-terminally located cluster of charged amino acid residues (KKEK) is crucial for the morphogenic activity of villin in vivo. In vitro experiments with a 22 amino acid synthetic peptide corresponding to this region of villin provide evidence that this motif is part of an F-actin-binding site that induces G-actin to polymerize. Chemical cross-linking of actin to this peptide, the effects of amino acid substitutions in peptides, and the behavior of villin variants further corroborate the participation of the KKEK sequence in actin contacts.