Snail1 transcriptional repressor binds to its own promoter and controls its expression

The product of Snail1 gene is a transcriptional repressor of E-cadherin expression and an inductor of the epithelial–mesenchymal transition in several epithelial tumour cell lines. Transcription of Snail1 is induced when epithelial cells are forced to acquire a mesenchymal phenotype. In this work we demonstrate that Snail1 protein limits its own expression: Snail1 binds to an E-box present in its promoter (at −146 with respect to the transcription start) and represses its activity. Therefore, mutation of the E-box increases Snail1 transcription in epithelial and mesenchymal cells. Evidence of binding of ectopic or endogenous Snail1 to its own promoter was obtained by chromatin immunoprecipitation (ChIP) experiments. Studies performed expressing different forms of Snail1 under the control of its own promoter demonstrate that disruption of the regulatory loop increases the cellular levels of Snail protein. These results indicate that expression of Snail1 gene can be regulated by its product and evidence the existence of a fine-tuning feed-back mechanism of regulation of Snail1 transcription

Snail1 expression is required for sarcomagenesis

Altres ajuts: Fundació La Marató de TV3 (120130)Snail1 transcriptional repressor is a major inducer of epithelial-to mesenchymal transition but is very limitedly expressed in adult animals. We have previously demonstrated that Snail1 is required for the maintenance of mesenchymal stem cells (MSCs), preventing their premature differentiation. Now, we show that Snail1 controls the tumorigenic properties of mesenchymal cells. Increased Snail1 expression provides tumorigenic capabilities to fibroblastic cells; on the contrary, Snail1 depletion decreases tumor growth. Genetic depletion of Snail1 in MSCs that are deficient in p53 tumor suppressor downregulates MSC markers and prevents the capability of these cells to originate sarcomas in immunodeficient SCID mice. Notably, an analysis of human sarcomas shows that, contrarily to epithelial tumors, these neoplasms display high Snail1 expression. This is particularly clear for undifferentiated tumors, which are associated with poor outcome. Together, our results indicate a role for Snail1 in the generation of sarcomas

Tyrosine Phosphorylation of Plakoglobin Causes Contrary Effects on Its Association with Desmosomes and Adherens Junction Components and Modulates β-Catenin-Mediated Transcription

Plakoglobin is a protein closely related to β-catenin that links desmosomal cadherins to intermediate filaments. Plakoglobin can also substitute for β-catenin in adherens junctions, providing a connection between E-cadherin and α-catenin. Association of β-catenin with E-cadherin and α-catenin is regulated by phosphorylation of specific tyrosine residues; modification of β-catenin Tyr654 and Tyr142 decreases binding to E-cadherin and α-catenin, respectively. We show here that plakoglobin can also be phosphorylated on tyrosine residues, but unlike β-catenin, this modification is not always associated with disrupted association with junctional components. Protein tyrosine kinases present distinct specificities on β-catenin and plakoglobin, and phosphorylation of β-catenin-equivalent Tyr residues of plakoglobin affects its interaction with components of desmosomes or adherens junctions differently. For instance, Src, which mainly phosphorylates Tyr86 in β-catenin, modifies Tyr643 in plakoglobin, decreasing the interaction with E-cadherin and α-catenin and increasing the interaction with the α-catenin-equivalent protein in desmosomes, desmoplakin. The tyrosine kinase Fer, which modifies β-catenin Tyr142, lessening its association with α-catenin, phosphorylates plakoglobin Tyr549 and exerts the contrary effect: it raises the binding of plakoglobin to α-catenin. These results suggest that tyrosine kinases like Src or Fer modulate desmosomes and adherens junctions differently. Our results also indicate that phosphorylation of Tyr549 and the increased binding of plakoglobin to components of adherens junctions can contribute to the upregulation of the transcriptional activity of the β-catenin-Tcf-4 complex observed in many epithelial tumor cells

Phosphorylation Regulates the Subcellular Location and Activity of the Snail Transcriptional Repressor

The Snail gene product is a transcriptional repressor of E-cadherin expression and an inducer of the epithelial-to-mesenchymal transition in several epithelial tumor cell lines. This report presents data indicating that Snail function is controlled by its intracellular location. The cytosolic distribution of Snail depended on export from the nucleus by a CRM1-dependent mechanism, and a nuclear export sequence (NES) was located in the regulatory domain of this protein. Export of Snail was controlled by phosphorylation of a Ser-rich sequence adjacent to this NES. Modification of this sequence released the restriction created by the zinc finger domain and allowed nuclear export of the protein. The phosphorylation and subcellular distribution of Snail are controlled by cell attachment to the extracellular matrix. Suspended cells presented higher levels of phosphorylated Snail and an augmented extranuclear localization with respect to cells attached to the plate. These findings show the existence in tumor cells of an effective and fine-tuning nontranscriptional mechanism of regulation of Snail activity dependent on the extracellular environment

Characteristics of 162 patients with colorectal cancer.

<p>Characteristics of 162 patients with colorectal cancer.</p

Expression of Snail1 in tumor and stroma according to tumor stage.

<p>Snail1 immunoreactivity was determined in the stromal or carcinoma cells corresponding to colorectal tumours classified in the different stages. According to Snail1 expression, tumours were classified as presenting Snail1 expression both in the tumour and stroma (T+/S+), just in the tumour (T+/S−), just in the stroma (T−/S+) or not present in either of these compartments (T−/S−).</p

Nuclear Snail1 protein expression in colon carcinomas.

<p>Expression of Snail protein was determined as indicated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005595#s4" target="_blank">Methods</a> in samples corresponding to colon carcinomas using MAb EC3. Micrographs of several representative stained sections are shown. Panels A–E corresponded to tumours considered positive only in the stroma; panel F, just in the tumour, and panels G–P; in both compartments. The arrow in panel H labels a cell that cannot be clearly classified as tumoral or stromal. In panel O the arrow points at a cell entering a vessel. Bars indicate magnification.</p

Specific survival of stage I, II and III colon tumour patients according to Snail1 expression in the stroma.

<p>The presence of Snail1 in the stroma of stage I, II and III tumours is represented as continuous lines; dotted lines correspond to stroma-negative tumours. In the lower left panel, expression of Snail1 in the stroma was considered as low or high according to the criteria indicated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005595#s4" target="_blank">Methods</a>. The significance is indicated in each category.</p