The transcription factor Snail downregulates the tight junction components independently of E-cadherin downregulation

Concordant correlation of LIV-1 and E-cadherin expression in human breast cancer cell MCF-7

The recent report highlighted a significant association between signal transducer and activator of transcription 3 (STAT3) and Snail and LIV-1 (SLC39A6 or ZIP6), the breast cancer-associated protein that belongs to a new subfamily of zinc transporters. LIV-1 is a downstream target of STAT3, both in zebrafish and mammalian cells and provides control over epithelial-mesenchymal transition (EMT). Crucially, these observations link LIV-1, previously demonstrated to be associated with lymph node metastasis in breast cancer, to genes with a proven role in development. A putative role of LIV-1 as a regulator of E-cadherin that modulates the cell-cell adhesion is thus inferred. In present study, the correlation of LIV-1 and E-cadherin expression in human breast cancer cell MCF-7 and the effect of LIV-1 expression on the cell growth were assessed to explore the possible mechanisms associated with this observation in breast cancer. It was shown that the silencing of LIV-1 would induce the down-expression of E-cadherin. There was opposite results if the cells were overexpressed with LIV-1. In addition, the results showed that promotion effect after silencing of LIV-1 and inhibition effect after overexpression of LIV-1 in transfected cells. To our knowledge, this is the first evidence that the expression of E-cadherin could be regulated by the zinc transporter LIV-1. The results suggest that there is an association of LIV-1 expression with less aggressive tumors due to high E-cadherin expression because of high LIV-1 expression. LIV-1 may be a regulator of E-cadherin.

The transcription factor snail represses Crumbs3 expression and disrupts apico-basal polarity complexes

In epithelial cells, the tight junction divides the plasma membrane into distinct apical and basolateral domains. Polarization is essential for epithelial cell function, and apico-basal cell polarity is lost during the epithelial to mesenchymal transition (EMT), a program of events characterized not only by loss of cell polarity, but also by enhanced cell motility and increased cell invasion. Among several apically localized protein complexes, the Crumbs and Par protein complexes have pivotal roles in control of epithelial polarity and apical membrane formation. Here, we demonstrate that the Snail transcriptional repressor antagonizes expression of the Crumbs polarity complex. We show that Snail abolishes localization of the Crumbs and Par complexes to the tight junction, decreases Crumbs complex protein levels and suppresses Crumbs3 transcription. Evidence that Snail acts directly to antagonize Crumbs3 promoter activity is presented. Strikingly, we note that reexpression of exogenous Crumbs3 in Snail-expressing Madin-Darby Canine Kidney cells partially restores cell-cell junctions. Moreover, we find that the EMT inducer transforming growth factor-beta elicits transcriptional repression of Crumbs3 and results in a measurable loss of Crumbs3 protein. Our findings provide new insights into the links between the transcriptional repression function of Snail and its role in antagonizing key apico-basal polarity factors during EMT.

The transcription factor snail mediates epithelial to mesenchymal transitions by repression of estrogen receptor-alpha

The estrogen receptor (ER)-alpha (ESR1) is a key regulatory molecule in mammary epithelial cell development. Loss of ER-alpha in breast cancer is correlated with poor prognosis, increased recurrence after treatment, and an elevated incidence of metastasis. A proposed molecular pathway by which ER-alpha acts to constrain invasive growth in breast cancer cells involves direct, ER-alpha-dependent expression of metastasis-associated protein 3, a cell-type-specific component of the Mi-2/NuRD chromatin remodeling complex. MTA3 in turn represses expression of Snail, a transcription factor linked to epithelial to mesenchymal transition and cancer metastasis. To elucidate its role(s) in epithelial to mesenchymal transition (EMT), we expressed Snail in the noninvasive, ER-alpha-positive MCF-7 cell line. Snail expression led to decreased cell-cell adhesion and increased cell invasiveness. Furthermore, we observed loss of ER-alpha expression at both the RNA and protein level that was accompanied by direct interaction of Snail with regulatory DNA sequences at the ESR1 locus. A consequence of loss of ER-alpha function in this system was the increased abundance of key components of the TGF-beta signaling pathway. Thus, cross-talk among ER-alpha, Snail, and the TGF-beta pathway appears to control critical phenotypic properties of breast cancer cells.

Function of the Zinc-Finger Transcription FactorSNAI2in Cancer and Development

Elucidation of the molecular mechanisms that underlie disease development is still a tremendous challenge for basic science, and a prerequisite to the development of new and disease-specific targeted therapies. This review focuses on the function of SNAI2, a member of the Snail family of zinc-finger transcription factors, and discusses its possible role in disease development. SNAI2 has been implicated in diseases of melanocyte development and cancer in humans. Many malignancies arise from a rare population of cells that alone have the ability to self-renew and sustain the tumor (i.e., cancer stem cells). SNAI2 controls key aspects of stem cell function in mouse and human, suggesting that similar mechanisms control normal development and cancer stem cell properties. These insights are expected to contribute significantly to the genetics of cancer and to the development of both cancer therapy and new methods for assessing treatment efficacy.

Reassessing epithelial to mesenchymal transition as a prerequisite for carcinoma invasion and metastasis

For most carcinomas, progression toward malignancy is accompanied by loss of epithelial differentiation and a shift towards a mesenchymal phenotype. This process, referred to as epithelial to mesenchymal transition (EMT), exacerbates motility and invasiveness of many cell types and is often considered a prerequisite for tumor infiltration and metastasis. However, there are numerous examples of advanced carcinomas that adopt some mesenchymal features, yet retain characteristics of well-differentiated epithelial cells. We provide a review of these reports and describe mechanisms to explain the morphologic and molecular heterogeneity and plasticity of malignant carcinoma cells, including incomplete EMT, reversion to an epithelial phenotype, and collective migration. We suggest that these mechanisms can manifest in a series of independent and reversible steps and that EMT represents just one mechanism in the global metastatic carcinoma development process.

Downregulation of Par-3 expression and disruption of Par complex integrity by TGF-beta during the process of epithelial to mesenchymal transition in rat proximal epithelial cells

Epithelial to mesenchymal transition (EMT) is a fundamental mechanism of organ fibrosis and the initial step is disruption of cell junction and cell polarity. TGF-beta has been demonstrated as the most important mediator of EMT which is sufficient to initiate and complete the whole course of EMT, however, the detailed mechanism of TGF-beta in modulating the disruption of cell junction still remains unclear. Par-3 is a component of Par complex which plays a crucial role in the establishment and maintenance of epithelial polarity. In this study, we found that TGF-beta treatment resulted in a dose- and time-dependent downregulation of Par-3 protein together with the suppression of E-cadherin expression and induction of alpha-SMA. The decreased Par-3 subsequently resulted in the redistribution of Par-6-aPKC complex from cell membrane to cytoplasm. Forced expression of exogenous Par-3 into rat proximal epithelial cells (NRK52E) led to a drastic blockage of TGF-beta1-induced E-cadherin suppression and alpha-SMA induction. In contrast, knockdown Par-3 expression by siRNA significantly enhanced TGF-beta1-induced E-cadherin suppression and alpha-SMA induction. These data indicate that downregulation of Par-3 and subsequent disruption of Par complex integrity might be one mechanism that TGF-beta destroys cell polarity during EMT.

Direct repression of cyclin D1 by SIP1 attenuates cell cycle progression in cells undergoing an epithelial mesenchymal transition

Zinc finger transcription factors of the Snail/Slug and ZEB-1/SIP1 families control epithelial-mesenchymal transitions in development in cancer. Here, we studied SIP1-regulated mesenchymal conversion of epidermoid A431 cells. We found that concomitant with inducing invasive phenotype, SIP1 inhibited expression of cyclin D1 and induced hypophosphorylation of the Rb tumor suppressor protein. Repression of cyclin D1 was caused by direct binding of SIP1 to three sequence elements in the cyclin D1 gene promoter. By expressing exogenous cyclin D1 in A431/SIP1 cells and using RNA interference, we demonstrated that the repression of cyclin D1 gene by SIP1 was necessary and sufficient for Rb hypophosphorylation and accumulation of cells in G1 phase. A431 cells expressing SIP1 along with exogenous cyclin D1 were highly invasive, indicating that SIP1-regulated invasion is independent of attenuation of G1/S progression. However, in another epithelial-mesenchymal transition model, gradual mesenchymal conversion of A431 cells induced by a dominant negative mutant of E-cadherin produced no effect on the cell cycle. We suggest that impaired G1/S phase progression is a general feature of cells that have undergone EMT induced by transcription factors of the Snail/Slug and ZEB-1/SIP1 families.

Changes in the distribution pattern of Claudin tight junction proteins during the progression of mouse skin tumorigenesis

Background

Despite the fact that morphological and physiological observations suggest that the tight junction (TJ)-based permeability barrier is modified/disrupted in tumorigenesis, the role of members of the Claudin (Cldn) family of TJ proteins is not well-understood. Using a well-established two-stage chemical carcinogenesis model, we investigated the temporal and spatial changes in expression of those Cldns that we have previously demonstrated to be important in epidermal differentiation and the formation of the epidermal permeability barrier, i.e., Cldn1, Cldn6, Cldn11, Cldn12 and Cldn18.

Methods

The lower dorsal backskin of mice was treated topically with 7,12-dimethylbenz(a)anthracene (DMBA; 0.25 mg/ml in acetone) and following a 10-day incubation period, 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 25 μg/ml in acetone) was applied three times a week to the same area. Backskin samples were dissected 2, 4, 6, 8 and 12 weeks after the initiation of the experimental protocol and immunohistochemistry was performed on sections using antibodies against the following: Cldn1, Cldn6, Cldn11, Cldn12, Cldn18, Ki67 and CD3.

Results

Our data indicate that along with the changes in epidermal cell morphology and differentiation that occur during tumor formation, there is a dramatic change in Cldn distribution consistent with cell polarity and barrier selectivity changes. Specifically, in the early stages of DMBA/TPA treatment, the suprabasal-specific Cldns occupy an expanded zone of expression corresponding to an increased number of suprabasal epidermal cell layers. As tumorigenesis progressed, the number of suprabasal epidermal layers positive for Cldn6, Cldn11, Cldn12 and Cldn18 was reduced, especially in the lower strata of the expanded suprabasal zone. In addition, a variably reduced cell membrane association of those differentiation-specific Cldns was observed, especially within the infiltrating epidermal structures. In contrast, Cldn1 (which is normally expressed in all the living layers of the epidermis) remained restricted to the cell membrane throughout the tumorigenesis protocol. However commencing 2 weeks after treatment there was a marked decrease in the number of Cldn1-positive basal cells, and the zone of Cldn1-null epidermal cells was expanded up into the lower stratified epidermis throughout the progression of DMBA/TPA treatment. In addition, there was no Cldn1 localization in the infiltrating epidermal structures of the tumorigenic epidermis.

Conclusion

This is the first demonstration of the changes in Cldn expression in the progression of DMBA/TPA-induced skin tumors; however further investigation into the molecular mechanisms regulating the observed changes in barrier selectivity during tumorigenesis is required.

Transmembrane proteins of tight junctions

Tight junctions contribute to the paracellular barrier, the fence dividing plasma membranes, and signal transduction, acting as a multifunctional complex in vertebrate epithelial and endothelial cells. The identification and characterization of the transmembrane proteins of tight junctions, claudins, junctional adhesion molecules (JAMs), occludin and tricellulin, have led to insights into the molecular nature of tight junctions. We provide an overview of recent progress in studies on these proteins and highlight their roles and regulation, as well as their functional significance in human diseases.

Expression of snail in pancreatic cancer promotes metastasis and chemoresistance

BACKGROUND

Pancreatic cancer is a dreadful malignancy. Because of its tendency to metastasis and its resistance to chemotherapy, the prognosis remains poor. Snail is a transcriptional factor that endows epithelial cells with migratory and anti-apoptotic abilities. Its expression has been demonstrated in many tumors. We hypothesized that Snail may be expressed in pancreatic cancer, and it may confer invasive and chemoresistant properties.

MATERIALS, METHODS, AND RESULTS

We immunohistochemically examined Snail expression in pancreatic cancer and found that it was expressed in 20 of 56 (36%) samples of pancreatic cancer. The Snail expression had a close correlation with lymph node invasion and distant metastasis. After transfecting Snail cDNA into pancreatic cancer cell line Panc-1, we found that Snail triggered overt epithelial to mesenchymal transitions in Panc-1 cells. The tumor invasive ability in vitro was evaluated using a transwell invasive chamber. Snail dramatically promoted the invasive ability of Panc-1 cells. Chemosensitivity of Panc-1 cells to 5-fluorouracil or gemcitabine after Snail transfection was assayed by MTT cell proliferation assay. Overexpression of Snail enhanced the chemoresistance to 5-fluorouracil of gemcitabine at different dosages. Moreover, Snail-transfected Panc-1 cells produced more spontaneous metastasis than parental untransfected cells after orthotopically injected into the pancreas of nude mice.

CONCLUSION

Snail is expressed in pancreatic cancer; it confers enhanced invasive ability and chemoresistance to pancreatic cancer cells. Snail may be a marker for predicting the malignancy of pancreatic cancer. Further therapy target to Snail may be of great benefit to pancreatic cancer patients.

Regulation of cell junction dynamics by cytokines in the testis: a molecular and biochemical perspective

Studies in the past decade in the field have demonstrated the significance of cytokines in regulating epithelial and endothelial cell junctions including tight and anchoring junctions in multiple organs including the testis. There are mounting evidences in recent years that cytokines play a crucial role in the restructuring of junctions at the Sertoli-Sertoli and Sertoli-germ cell interface in the seminiferous epithelium during spermatogenesis. These earlier studies, however, were focused on the effects of cytokines in maintaining the steady-state protein levels of integral membrane proteins at the sites of the blood-testis barrier (BTB) and anchoring junctions at the Sertoli-Sertoli and Sertoli-germ cell interface, such as basal and apical ectoplasmic specialization, respectively. The molecular pathway(s) and/or mechanism(s) underlying these effects remained virtually unexplored until very recently. Herein, we summarize and provide some discussions on studies that focused on the role of cytokines in regulating junction restructuring events in epithelia from a molecular and biochemical perspective. Specifically, we use the adult rat or mouse testis as a model to highlight the significance of transcriptional and translational regulation. Specific areas of research that require further attentions are also highlighted.

Id-1 is induced in MDCK epithelial cells by activated Erk/MAPK pathway in response to expression of the Snail and E47 transcription factors

Id-1, a member of the helix-loop-helix transcription factor family has been shown to be involved in cell proliferation, angiogenesis and invasion of many types of human cancers. We have previously shown that stable expression of E47 and Snail repressors of the E-cadherin promoter in MDCK epithelial cell line triggers epithelial mesenchymal transition (EMT) concomitantly with changes in gene expression. We show here that both factors activate the Id-1 gene promoter and induce Id-1 mRNA and protein. The upregulation of the Id-1 gene occurs through the transactivation of the promoter by the Erk/MAPK signaling pathway. Moreover, oncogenic Ras is also able to activate Id-1 promoter in MDCK cells in the absence of both E47 and Snail transcription factors. Several transcriptionally active regulatory elements have been identified in the proximal promoter, including AP-1, Sp1 and four putative E-boxes. By EMSA, we only detected an increased binding to Sp1 and AP-1 elements in E47- and Snail-expressing cells. Binding is affected by the treatment of cells with PD 98059 MEK inhibitor, suggesting that MAPK/Erk contributes to the recruitment or assembly of proteins to Id-1 promoter. Small interfering RNA directed against Sp1 reduced Id-1 expression and the upregulation of the promoter, indicating that Sp1 is required for Id-1 induction in E47- and Snail-expressing cells. Our results provide new insights into how some target genes are activated during and/or as a consequence of the EMT triggered by both E47 and Snail transcription factors.

Hypoxia disrupts the barrier function of neural blood vessels through changes in the expression of claudin-5 in endothelial cells

The mechanisms underlying the hypoxia-induced disruption of the barrier function of neural vasculature were analyzed with reference to the expression of claudin-5, a component of tight junctions between neural endothelial cells. The movement of claudin-5 from the cytoplasm to the plasma membrane of cultured confluent brain-derived endothelial (bEND.3) cells was closely correlated with the increase in the transendothelial electrical resistance. Inhibition of the expression of claudin-5 by RNAi resulted in a reduction of transendothelial electrical resistance, indicating a critical role of claudin-5 in the barrier property. Hypoxia (1% O(2)) altered the location of claudin-5 in the plasma membrane and the level of claudin-5 protein in bEND.3 cells, and these changes were accompanied by a decrease in the transendothelial electrical resistance. In vivo the claudin-5 molecules were expressed under normoxia in the plasma membrane of retinal microvascular endothelial cells but were significantly reduced under hypoxic conditions. Tracer experiments revealed that the barrier function of hypoxic retinal vasculature with depressed claudin-5 expression was selectively disrupted against small molecules, which is very similar to the phenotype of claudin-5-deficient mice. These in vitro and in vivo data indicate that claudin-5 is a target molecule of hypoxia leading to the disruption of the barrier function of neural vasculature.

Snail, a transcriptional regulator, represses nephrin expression in glomerular epithelial cells of nephrotic rats

Snail is a DNA-binding molecule that plays a pivotal role in regulating cell adhesion and epithelial to mesenchymal transition. Visceral epithelial cells (podocytes) in kidney glomeruli form a sophisticated cell-cell junction called a slit diaphragm that prevents the loss of plasma protein during ultrafiltration. Nephrin, located in the slit diaphragm and critical for maintaining the integrity of this structure, belongs to the class of cell adhesion molecules of the immunoglobulin super-family. As previously reported, the transcriptional activity of nephrin is a determinant of the integrity of the slit diaphragm in puromycin aminonucleoside (PAN) nephrosis rats. Here, we examined the role of Snail in nephrin expression. In accordance with the downregulation of nephrin in PAN nephrosis rats, Snail was upregulated in vivo and its DNA-binding activity was stimulated in injured podocytes while normal podocytes did not express Snail. An in vitro study demonstrated that Snail bound to E-box motifs in a specific segment of the rat nephrin gene repressed the transcription of nephrin and downregulated nephrin protein. We also found that the expression level of Snail in injured podocytes was regulated by GSK3, which is known to phosphorylate Snail and induce its proteolysis. Pharmacological in vitro and in vivo inhibition studies of GSK3 suggested that GSK3 activity decreased in injured podocytes and this change partially contributed to the decrease in nephrin and increase in Snail and proteinuria. Concordantly, we found that Wnt-2 was upregulated in injured podocytes and activated the Wnt canonical pathway. As the Wnt canonical pathway inactivates GSK3, it is likely that Wnt-2 accounts for the accumulation of Snail in injured podocytes. In conclusion, Snail is a key molecule, which perturbs the integrity of the slit diaphragm through transcriptional repression of nephrin under pathological conditions. Wnt-GSK3 pathway participates in this mechanism.

Role of tight junctions in cell proliferation and cancer

The acquisition of a cancerous phenotype by epithelial cells involves the disruption of intercellular adhesions. The reorganization of the E-cadherin/beta-catenin complex in adherens junctions during cell transformation is widely recognized. Instead the implication of tight junctions (TJs) in this process is starting to be unraveled. The aim of this article is to review the role of TJ proteins in cell proliferation and cancer.

Activation of an SP binding site is crucial for the expression of claudin 1 in rat epididymal principal cells

Claudin 1 (CLDN1) is a tight junctional protein present in the epididymis. Limited information exists regarding the regulation of Cldn1 transcription. In the epididymis, the regulation of the 5' flanking region of genes coding for tight junctional proteins is unknown. The present objectives were to investigate the transcriptional regulation of the Cldn1 gene in the rat epididymis. A 1.8-kb sequence of the 5' flanking region of the rat Cldn1 gene was cloned. The transcriptional start site is an adenine located at the -198 position relative to the first codon, and 26 bp downstream of the putative TATA box. It is the only start site for the Cldn1 gene transcription in the rat epididymis. The Cldn1 promoter was inserted into a luciferase gene expression vector and transfected into a rat caput epididymal cell line (RCE-1). Sequential deletion analysis revealed that minimal promoter activity was achieved with the construct containing -61 to +164 bp of the promoter. This sequence contained a TATA box and two consensus SP1 binding sites. Electrophoretic mobility shift and supershift assays confirmed that SP1 and SP3 were present in RCE-1 cells and epididymal nuclear extracts, and that they bind to the 5' SP1 binding motif of the promoter. Site-directed mutagenesis of the 5' SP1 binding site resulted in a 4-fold decrease in transactivation of the minimal promoter sequence. These findings indicate that SP1 and SP3 bind to the Cldn1 promoter region, and that this interaction influences the expression of Cldn1 in the rat epididymis.

Plakophilins: Multifunctional proteins or just regulators of desmosomal adhesion?

Plakophilins 1-3 are members of the p120(ctn) family of armadillo-related proteins. The plakophilins have been characterized as desmosomal proteins, whereas p120(ctn) and the closely related delta-catenin, ARVCF and p0071 associate with adherens junctions and play essential roles in stabilizing cadherin mediated adhesion. Recent evidence suggests that plakophilins are essential components of the desmosomal plaque where they interact with desmosomal cadherins as well as the cytoskeletal linker protein desmoplakin. Plakophilins stabilize desmosomal proteins at the plasma membrane and therefore may function in a manner similar to p120(ctn) in the adherens junctions. The three plakophilins reveal distinct expression patterns, and although partially redundant in their function, mediate distinct effects on desmosomal adhesion. Besides a structural role, a function in signaling has been postulated in analogy to other armadillo proteins such as beta-catenin. At least plakophilins 1 and 2 are also localized in the nucleus, and all three proteins occur in a cytoplasmic pool. This review aims to summarize the current knowledge of plakophilin function in the context of cell adhesion, signaling and their putative role in diseases.

Expression of E-cadherin and its repressor Snail in placental tissue of normal, preeclamptic and HELLP pregnancies

Incomplete invasion of extravillous trophoblasts (EVT) is thought to be associated with complications of pregnancy. Snail, a zinc-finger transcription factor represses the transcription of the cell adhesion molecule E-cadherin. The aim of this study was to investigate the expression of E-cadherin and Snail in placental tissue with preeclampsia or HELLP. Placental tissues were obtained from five patients with HELLP syndrome, seven patients with preeclampsia and seven patients after a normal term birth and analysed for Snail and E-cadherin immunoreactivity with specific monoclonal antibodies. Immunohistochemical staining of the placental tissue was analysed using an immunoreactivity score for the evaluation of staining intensity. In preeclamptic EVT, Snail immunoreactivity showed a significant 1.7-fold increase, accompanied by a significant 1.9-fold reduction of E-cadherin immunoreactivity. A 1.7-fold increase of Snail and in parallel a 1.3-fold reduction of E-cadherin was observed in EVT of HELLP placentas although without statistical significance. Loss of E-cadherin can be observed during epithelial-mesenchymal transition (EMT), a key process in normal development and trophoblast differentiation. Snail represses the transcription of E-cadherin triggering a complete EMT. Results obtained in this study showed changes of Snail and E-cadherin immunoreactivity in preeclamptic placentas that could be accompanied with an altered EMT in trophoblasts.

Epithelial-mesenchymal transformation during craniofacial development

Epithelial to mesenchymal phenotype transition is a common phenomenon during embryonic development, wound healing, and tumor metastasis. This transition involves cellular changes in cytoskeleton architecture and protein expression. Specifically, this highly regulated biological event plays several important roles during craniofacial development. This review focuses on the regulation of epithelial-mesenchymal transformation (EMT) during neural crest cell migration, and fusion of the secondary palate and the upper lip.

Snail silencing effectively suppresses tumour growth and invasiveness

The transcription factor Snail has been recently proposed as an important mediator of tumour invasion because of its role in downregulation of E-cadherin and induction of epithelial-mesenchymal transitions (EMT). This behaviour has led to the consideration of Snail as a potential therapeutic target to block tumour progression. In this report, we provide evidence for this hypothesis. We show that silencing of Snail by stable RNA interference in MDCK-Snail cells induces a complete mesenchymal to epithelial transition (MET), associated to the upregulation of E-cadherin, downregulation of mesenchymal markers and inhibition of invasion. More importantly, stable interference of endogenous Snail in two independent carcinoma cell lines leads to a dramatic reduction of in vivo tumour growth, accompanied by increased tumour differentiation and a significant decrease in the expression of MMP-9 and angiogenic markers and invasiveness. These results indicate that use of RNA interference can be an effective tool for blocking Snail function, opening the way for its application in new antiinvasive therapies.

Calreticulin Represses E-cadherin Gene Expression in Madin-Darby Canine Kidney Cells via Slug

Calreticulin (CRT) is a multifunctional Ca(2+)-binding molecular chaperone in the endoplasmic reticulum. In mammals, the expression level of CRT differs markedly in a variety of organs and tissues, suggesting that CRT plays a specific role in each cell type. In the present study, we focused on CRT functions in the kidney, where overall expression of CRT is quite low, and established CRT-overexpressing kidney epithelial cell-derived Madin-Darby canine kidney cells by gene transfection. We demonstrated that, in CRT-overexpressing cells, the morphology was apparently changed, and the original polarized epithelial cell phenotype was destroyed. Furthermore, CRT-overexpressing cells showed enhanced migration through Matrigel-coated Boyden chamber wells, compared with controls. E-cadherin expression was significantly suppressed at the protein and transcriptional levels in CRT-overexpressing cells compared with controls. On the other hand, the expression of mesenchymal protein markers, such as N-cadherin and fibronectin, was up-regulated. We also found that the expression of Slug, a repressor of the E-cadherin promoter, was up-regulated by overexpression of CRT through altered Ca(2+) homeostasis, and this led to enhanced binding of Slug to the E-box element in the E-cadherin promoter. Thus, we conclude that CRT regulates the epithelial-mesenchymal transition-like change of cellular phenotype by modulating the Slug/E-cadherin pathway through altered Ca(2+) homeostasis in cells, suggesting a novel function of CRT in cell-cell interaction of epithelial cells.

Diverse functions of p120ctn in tumors

p120ctn is a member of the Armadillo protein family. It stabilizes the cadherin-catenin adhesion complex at the plasma membrane, but also has additional roles in the cytoplasm and nucleus. Extensive alternative mRNA splicing and multiple phosphorylation sites generate additional complexity. Evidence is emerging that complete loss, downregulation or mislocalization of p120ctn correlates with progression of different types of human tumors. It remains to be determined whether a causal relationship exists between specific isoform expression, subcellular localization or selective phosphorylation of p120ctn on the one hand and tumor prognosis on the other.

SPARC represses E-cadherin and induces mesenchymal transition during melanoma development

During progression of melanoma, loss of the cell-cell adhesion molecule E-cadherin contributes to uncontrolled growth and invasive behavior of transformed melanocytes. Secreted protein acidic and rich in cysteine (SPARC) is a nonstructural matricellular protein that regulates cell-matrix interactions leading to alterations in cell adhesion and proliferation. Overexpression of SPARC has been associated with progression of various cancers, including melanoma; however, its role in primary tumor development is not well defined. We show that normal human melanocytes overexpressing SPARC adopt a fibroblast-like morphology, concomitant with loss of E-cadherin and P-cadherin expression, and increased expression of mesenchymal markers. Concurrent with these changes, SPARC expression stimulates melanocyte motility and melanoma cell invasion. Expression of SPARC results in transcriptional down-regulation of E-cadherin that correlates with induction of Snail, a repressor of E-cadherin. Conversely, SPARC depletion leads to up-regulation of E-cadherin and reduces Snail levels, and SPARC-null cells exhibit a marked change in their mesenchymal phenotype. Finally, analysis of SPARC, Snail, and E-cadherin levels in melanocytes and malignant melanoma cell lines further supports the functional relationship among these proteins during melanoma progression. Our findings provide evidence for the role of SPARC in early transformation of melanocytes and identify a novel mechanism, whereby tumor-derived SPARC promotes tumorigenesis by mediating Snail induction and E-cadherin suppression.

Expression of inappropriate cadherins by epithelial tumor cells promotes endocytosis and degradation of E-cadherin via competition for p120(ctn)

Cadherin cell-cell adhesion proteins play an important role in modulating the behavior of tumor cells. E-cadherin serves as a suppressor of tumor cell invasion, and when tumor cells turn on the expression of a non-epithelial cadherin, they often express less E-cadherin, enhancing the tumorigenic phenotype of the cells. Here, we show that when A431 cells are forced to express R-cadherin, they dramatically downregulate the expression of endogenous E- and P-cadherin. In addition, we show that this downregulation is owing to increased turnover of the endogenous cadherins via clathrin-dependent endocytosis. p120(ctn) binds to the juxtamembrane domain of classical cadherins and has been proposed to regulate cadherin adhesive activity. One way p120(ctn) may accomplish this is to serve as a rheostat to regulate the levels of cadherin. Here, we show that the degradation of E-cadherin in response to expression of R-cadherin is owing to competition for p120(ctn).

Claudins and epithelial paracellular transport

Tight junctions form continuous intercellular contacts controlling solute movement through the paracellular pathway across epithelia. Paracellular barriers vary among epithelia in electrical resistance and behave as if they are lined with pores that have charge and size selectivity. Recent evidence shows that claudins, a large family (at least 24 members) of intercellular adhesion molecules, form the seal and its variable pore-like properties. This evidence comes from the study of claudins expressed in cultured epithelial cell models, genetically altered mice, and human mutants. We review information on the structure, function, and transcriptional and posttranslational regulation of the claudin family as well as of their evolutionarily distant relatives called the PMP22/EMP/MP20/claudin, or pfam00822, superfamily.

Transcription factor snail modulates hormone expression in established endocrine pancreatic cell lines

The development of differentiated cells from undifferentiated progenitor cells is one of the central tenets of developmental biology. However, under conditions of tissue morphogenesis, regeneration, and cancer, this process of development is reversed and fully differentiated cells transition to an undifferentiated phenotype. Here we present evidence that the zinc-finger transcription factor Snail modulates this transition in differentiated pancreatic endocrine cell lines. During passage and growth of these cell lines, Snail expression is induced in a subset of cells within the culture, concomitant with a decrease in insulin and/or glucagon expression. As the cells cluster and exit the cell division cycle, nuclear levels of Snail are reduced and hormone expression is resumed. Snail represses proinsulin and proglucagon gene transcription, and reduction of Snail levels by small interfering RNA treatment increases proinsulin gene expression. We propose that Snail modulates the dynamic balance between differentiated and dedifferentiated cells allowing their migration and proliferation. These findings may be relevant to providing approaches for the enhancement of beta-cell growth in individuals with diabetes mellitus.

Regulation of junction dynamics in the testis--transcriptional and post-translational regulations of cell junction proteins

Cell junctions are the sites at which cells attach to the neighboring cells. They do not only maintain tissue integrity, their turnover also plays a crucial role in cell development and morphogenesis. In the testis, tight junctions and adherens junctions are dynamically remodeled to allow the movement of post-meiotic germ cells across the seminiferous epithelium and the timely release of spermatids into the tubular lumen. There is growing evidence that this dynamic remodeling of cell junctions is mediated by several mechanisms at the transcriptional and post-translational levels. This review summarizes what is known about the transcriptional regulation, ubiquitination and endocytosis that are involved in modulating junction dynamics in epithelial cells. It also highlights the recent findings on the regulation of junction dynamics in the testis and the specific areas that require further research for a thorough understanding of the role of junction remodeling in spermatogenesis. Understanding the junction dynamics in the seminiferous epithelium may unfold new targets for non-hormonal male contraceptive development.

The transcription factors Slug and Snail act as repressors of Claudin-1 expression in epithelial cells

Claudin-1 is an integral membrane protein component of tight junctions. The Snail family of transcription factors are repressors that play a central role in the epithelial-mesenchymal transition, a process that occurs during cancer progression. Snail and Slug members are direct repressors of E-cadherin and act by binding to the specific E-boxes of its proximal promoter. In the present study, we demonstrate that overexpression of Slug or Snail causes a decrease in transepithelial electrical resistance. Overexpression of Slug and Snail in MDCK (Madin-Darby canine kidney) cells down-regulated Claudin-1 at protein and mRNA levels. In addition, Snail and Slug are able to effectively repress human Claudin-1-driven reporter gene constructs containing the wild-type promoter sequence, but not those with mutations in two proximal E-box elements. We also demonstrate by band-shift assay that Snail and Slug bind to the E-box motifs present in the human Claudin-1 promoter. Moreover, an inverse correlation in the levels of Claudin-1 and Slug transcripts were observed in breast cancer cell lines. E-box elements in the Claudin-1 promoter were found to play a critical negative regulatory role in breast cancer cell lines that expressed low levels of Claudin-1 transcript. Significantly, in invasive human breast tumours, high levels of Snail and Slug correlated with low levels of Claudin-1 expression. Taken together, these results support the hypothesis that Claudin-1 is a direct downstream target gene of Snail family factors in epithelial cells.

Regulation of vimentin by SIP1 in human epithelial breast tumor cells

The expression of Smad interacting protein-1 (SIP1; ZEB2) and the de novo expression of vimentin are frequently involved in epithelial-to-mesenchymal transitions (EMTs) under both normal and pathological conditions. In the present study, we investigated the potential role of SIP1 in the regulation of vimentin during the EMT associated with breast tumor cell migration and invasion. Examining several breast tumor cell lines displaying various degrees of invasiveness, we found SIP1 and vimentin expression only in invasive cell lines. Also, using a model of cell migration with human mammary MCF10A cells, we showed that SIP1 is induced specifically in vimentin-positive migratory cells. Furthermore, transfection of SIP1 cDNA in MCF10A cells increased their vimentin expression both at the mRNA and protein levels and enhanced their migratory abilities in Boyden Chamber assays. Inversely, inhibition of SIP1 expression by RNAi strategies in BT-549 cells and MCF10A cells decreased vimentin expression. We also showed that SIP1 transfection did not activate the TOP-FLASH reporter system, suggesting that the beta-catenin/TCF pathway is not implicated in the regulation of vimentin by SIP1. Our results therefore implicate SIP1 in the regulation of vimentin observed in the EMT associated with breast tumor cell migration, a pathway that may contribute to the metastatic progression of breast cancer.

Zinc transport activity of Fear of Intimacy is essential for proper gonad morphogenesis and DE-cadherin expression

Embryonic gonad formation involves intimate contact between germ cells and specialized somatic cells along with the complex morphogenetic movements necessary to create proper gonad architecture. Previously, we have shown that gonad formation in Drosophila requires the homophilic cell-adhesion molecule Drosophila E-cadherin (DE-cadherin), and also Fear of Intimacy (FOI), which is required for stable accumulation of DE-cadherin protein in the gonad. Here, we present an in vivo structure-function analysis of FOI that strongly indicates that zinc transport activity of FOI is essential for gonad development. Mutant forms of FOI that are defective for zinc transport also fail to rescue morphogenesis and DE-cadherin expression in the gonad. We further show that expression of DE-cadherin in the gonad is regulated post-transcriptionally and that foi affects this post-transcriptional control. Expression of DE-cadherin from a ubiquitous(tubulin) promoter still results in gonad-specific accumulation of DE-cadherin, which is strongly reduced in foi mutants. This work indicates that zinc is a crucial regulator of developmental processes and can affect DE-cadherin expression on multiple levels.

Snail induction is an early response to Gli1 that determines the efficiency of epithelial transformation

Gli family members mediate constitutive Hedgehog signaling in the common skin cancer, basal cell carcinoma (BCC). Snail/Snai1 is rapidly induced by Gli1 in vitro, and is coexpressed with Gli1 in human hair follicles and skin tumors. In the current study, we generated a dominant-negative allele of Snail, SnaZFD, composed of the zinc-finger domain and flanking sequence. In promoter-reporter assays, SnaZFD blocked the activity of wild-type Snail on the E-cadherin promoter. Snail loss-of-function mediated by SnaZFD or by one of several short hairpin RNAs inhibited transformation of RK3E epithelial cells by Gli1. Conversely, enforced expression of Snail promoted transformation in vitro by Gli1, but not by other genes that were tested, including Notch1, ErbB2, and N-Ras. As observed for Gli1, wild-type Snail repressed E-cadherin in RK3E cells and induced blebbing of the cytoplasmic membrane. Induction of a conditional Gli1 transgene in the basal keratinocytes of mouse skin led to rapid upregulation of Snail transcripts and to cell proliferation in the interfollicular epidermis. Established Gli1-induced skin lesions exhibited molecular similarities to BCC, including loss of E-cadherin. The results identify Snail as a Gli1-inducible effector of transformation in vitro, and an early Gli1-responsive gene in the skin.

Transcription repressor slug promotes carcinoma invasion and predicts outcome of patients with lung adenocarcinoma

PURPOSE

In a previous genome-wide gene expression profiling analysis using an invasion cancer cell lines model, we have identified Slug as selectively overexpressed in the highly invasive cancer cells. Here, we investigated the clinical significance of Slug in lung adenocarcinoma and the role of Slug in the process of cancer cell invasion and metastasis.

EXPERIMENTAL DESIGN

Real-time quantitative reverse transcription-PCR was used to investigate Slug mRNA in surgically resected lung adenocarcinoma of 54 patients and its correlation with survival. We overexpressed Slug in a lung adenocarcinoma cell line with very low Slug levels and investigated the in vitro and in vivo effects of Slug expression.

RESULTS

High expression of Slug mRNA in lung cancer tissue was significantly associated with postoperative relapse (P = 0.03) and shorter patient survival (P < 0.001). The overexpression of Slug enhanced xenograft tumor growth and increased microvessel counts in angiogenesis assay. Both inducible and constitutive overexpression of Slug suppressed the expression of E-cadherin and increased the in vitro invasive ability. Zymography revealed increased matrix metalloproteinase-2 activity in Slug overexpressed cells. ELISA, reverse transcription-PCR, and immunohistochemistry confirmed the increase of matrix metalloproteinase-2 proteins and mRNA in Slug overexpressed cells and xenograft tumors.

CONCLUSIONS

Slug expression can predict the clinical outcome of lung adenocarcinoma patients. Slug is a novel invasion-promoting gene in lung adenocarcinoma.

Cooperation between snail and LEF-1 transcription factors is essential for TGF-beta1-induced epithelial-mesenchymal transition

Transforming growth factor beta 1 (TGF-beta1) has been shown to induce epithelial-mesenchymal transition (EMT) during various stages of embryogenesis and progressive disease. This alteration in cellular morphology is typically characterized by changes in cell polarity and loss of adhesion proteins such as E-cadherin. Here we demonstrate that EMT is associated with loss of claudin-1, claudin-2, occludin, and E-cadherin expression within 72 h of exposure to TGF-beta1 in MDCKII cells. It has been suggested that this expression loss occurs through TGF-beta1 in a Smad-independent mechanism, involving MEK and PI3K pathways, which have previously been shown to induce expression of the Snail (SNAI-1) gene. Here we show that these pathways are responsible for loss of tight junctions and a partial loss of E-cadherin. However, our results also demonstrate that a complete loss of E-cadherin and transformation to the mesenchymal phenotype are dependent on Smad signaling, which subsequently stimulates formation of beta-catenin/LEF-1 complexes that induce EMT.

Differential expression of transcriptional repressor snail gene at implantation site in mouse uterus

The snail superfamily of zinc-finger transcription factors is involved in pronounced cell movements during both embryonic development and tumor progression. This study was to examine snail expression in mouse uterus during early pregnancy and its regulation under pseudopregnancy, delayed implantation, steroid hormone treatment, and artificial decidualization by in situ hybridization and immunohistochemistry. There was a low level of snail mRNA signal and immunostaining in mouse uteri on day 1-4 of pregnancy. When embryo implanted on day 5, both snail mRNA signal and immunostaining were strongly detected in the subluminal stroma immediately surrounding the implanting blastocyst, but not detected in the inter-implantation sites. Under delayed implantation, there was no detectable snail expression. After delayed implantation was terminated by estrogen treatment and embryo implanted, there was a strong level of snail mRNA and immunostaining in the subluminal stroma surrounding the implanting blastocyst, which was similar to that on day 5 of pregnancy. Furthermore, there was no detectable snail expression in mouse uterus on day 5 of pseudopregnancy. From day 6-8 of pregnancy, both snail mRNA signal and immunostaining were detected in the decidua. Our data suggest that snail may play an important role during mouse embryo implantation.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

Mouse cDNA microarray analysis uncovers Slug targets in mouse embryonic fibroblasts

There is a need to reveal mechanisms that account for maintenance of the mesenchymal phenotype in normal development and cancer. Slug (approved gene symbol Snai2), a member of the Snail gene family of zinc-finger transcription factors, is believed to function in the maintenance of the nonepithelial phenotype. This study identified candidate Slug target genes linked to Slug gene suppression in primary mouse embryonic fibroblasts. Expression analyses were performed with a mouse cDNA microarray (Mousechip-CNIO) containing 15,000 clones. A total of 15 novel Slug target species were validated by real-time PCR or Western analyses. These included self-renewal genes (Bmi1, Nanog, Gfi1), epithelial-mesenchymal genes (Tcfe2a, Ctnb1, Sin3a, Hdac1, Hdac2, Muc1, Cldn11), survival genes (Bcl2, Bbc3), and cell cycle/damage genes (Cdkn1a, Rbl1, Mdm2). Expression patterns were studied in wild-type MEFs and Slug-deficient MEFs. Slug-complementation studies recovered aberrant gene expression in cells lacking Slug, indicating that these genes were regulated directly by Slug. These results highlight their potential roles in mediating Slug function in mesenchymal cells and may help to identify novel therapeutic biomarkers in cancers linked to Slug.

SIP1/ZEB2 induces EMT by repressing genes of different epithelial cell-cell junctions

SIP1/ZEB2 is a member of the deltaEF-1 family of two-handed zinc finger nuclear factors. The expression of these transcription factors is associated with epithelial mesenchymal transitions (EMT) during development. SIP1 is also expressed in some breast cancer cell lines and was detected in intestinal gastric carcinomas, where its expression is inversely correlated with that of E-cadherin. Here, we show that expression of SIP1 in human epithelial cells results in a clear morphological change from an epithelial to a mesenchymal phenotype. Induction of this epithelial dedifferentiation was accompanied by repression of several cell junctional proteins, with concomitant repression of their mRNA levels. Besides E-cadherin, other genes coding for crucial proteins of tight junctions, desmosomes and gap junctions were found to be transcriptionally regulated by the transcriptional repressor SIP1. Moreover, study of the promoter regions of selected genes by luciferase reporter assays and chromatin immunoprecipitation shows that repression is directly mediated by SIP1. These data indicate that, during epithelial dedifferentiation, SIP1 represses in a coordinated manner the transcription of genes coding for junctional proteins contributing to the dedifferentiated state; this repression occurs by a general mechanism mediated by Smad Interacting Protein 1 (SIP1)-binding sites.

Immediate and delayed effects of E-cadherin inhibition on gene regulation and cell motility in human epidermoid carcinoma cells

The invasion suppressor protein, E-cadherin, plays a central role in epithelial cell-cell adhesion. Loss of E-cadherin expression or function in various tumors of epithelial origin is associated with a more invasive phenotype. In this study, by expressing a dominant-negative mutant of E-cadherin (Ec1WVM) in A431 cells, we demonstrated that specific inhibition of E-cadherin-dependent cell-cell adhesion led to the genetic reprogramming of tumor cells. In particular, prolonged inhibition of cell-cell adhesion activated expression of vimentin and repressed cytokeratins, suggesting that the effects of Ec1WVM can be classified as epithelial-mesenchymal transition. Both short-term and prolonged expression of Ec1WVM resulted in morphological transformation and increased cell migration though to different extents. Short-term expression of Ec1WVM up-regulated two AP-1 family members, c-jun and fra-1, but was insufficient to induce complete mesenchymal transition. AP-1 activity induced by the short-term expression of Ec1WVM was required for transcriptional up-regulation of AP-1 family members and down-regulation of two other Ec1WVM-responsive genes, S100A4 and igfbp-3. Using a dominant-negative mutant of c-Jun (TAM67) and RNA interference-mediated silencing of c-Jun and Fra-1, we demonstrated that AP-1 was required for cell motility stimulated by the expression of Ec1WVM. In contrast, Ec1WVM-mediated changes in cell morphology were AP-1-independent. Our data suggest that mesenchymal transition induced by prolonged functional inhibition of E-cadherin is a slow and gradual process. At the initial step of this process, Ec1WVM triggers a positive autoregulatory mechanism that increases AP-1 activity. Activated AP-1 in turn contributes to Ec1WVM-mediated effects on gene expression and tumor cell motility. These data provide novel insight into the tumor suppressor function of E-cadherin.

Inducible expression of Snail selectively increases paracellular ion permeability and differentially modulates tight junction proteins

Constitutive expression of the transcription factor Snail was previously shown to trigger complete epithelial-mesenchymal transition (EMT). The aim of this study was to determine whether inducible expression of Snail could modify epithelial properties without eliciting full mesenchymal conversion. For this purpose, we expressed mouse Snail (mSnail) cDNA in Madin-Darby canine kidney (MDCK) cells under the control of a doxycycline-repressible transactivator. Inducible expression of Snail did not result in overt EMT but induced a number of phenotypic alterations of MDCK cells, the most significant of which was the absence of fluid-filled blisterlike structures called “domes.” To understand the mechanisms responsible for dome suppression, we assessed the effect of mSnail expression on epithelial barrier function. Although mSnail did not alter tight junction (TJ) organization and permeability to uncharged solutes, it markedly decreased transepithelial electrical resistance. In light of these findings, we evaluated the ability of MDCK cell monolayers to maintain ionic gradients and found that expression of mSnail selectively increases Na+and Cl−permeability. Analysis of the expression of claudins, transmembrane proteins that regulate TJ ionic permeability, showed that mSnail induces a moderate decrease in claudin-2 and a substantial decrease in claudin-4 and -7 expression. Together, these results suggest that induction of mSnail selectively increases the ionic permeability of TJs by differentially modulating the expression of specific claudins.

The transcription factor snail induces tumor cell invasion through modulation of the epithelial cell differentiation program

Abberant activation of the process of epithelial-mesenchymal transition in cancer cells is a late event in tumor progression. A key inducer of this transition is the transcription factor Snail, which represses E-cadherin. We report that conditional expression of the human transcriptional repressor Snail in colorectal cancer cells induces an epithelial dedifferentiation program that coincides with a drastic change in cell morphology. Snail target genes control the establishment of several junctional complexes, intermediate filament networks, and the actin cytoskeleton. Modulation of the expression of these genes is associated with loss of cell aggregation and induction of invasion. Chromatin immunoprecipitation experiments showed that repression of selected target genes is associated with increased binding of Snail to their promoters, which contain consensus Snail-binding sites. Thus, Snail constitutes a master switch that directly represses the epithelial phenotype, resulting in malignant carcinoma cells.

Snail and Slug are major determinants of ovarian cancer invasiveness at the transcription level

OBJECTIVES

The transcriptional factors Snail and Slug have been reported to be important in cell migration during development and also during tumor metastasis. Their expression and role in ovarian cancer, hitherto unexplored, was examined to understand the molecular events in ovarian cancer metastases since the latter is responsible for the high degree of mortality associated with the disease.

METHODS

Ectopic expression of mSnail and mSlug in the epithelial ovarian cancer cell line SKOV3 was carried out and stable clones were selected. These were used to examine specific repression of the adherens, tight and desmosomal junction components by the two transcription factors. Furthermore, functional implications with respect to enhanced migration of cells, tumorigenecity and metastasis were also studied.

RESULTS

The ectopic expression of Snail or Slug resulted in epithelial-mesenchymal transition (EMT), enhanced motility, invasiveness and tumorigenecity in the cell line SKOV3. In addressing the mechanism by which Snail and Slug lead to loss of intercellular adhesion, specific repression of adherens junction components (E-cadherin and betacatenin), tight junction components (Occludin and ZO-1) and desmosomal junction components (Dsg2) were observed. Snail suppresses expression of adherens and tight junction components, while Slug suppresses expression of all the three junction components; concertedly, bringing down the intercellular adhesion between cells. Further activation of these transcriptional factors in hypoxic conditions revealed a rapid upregulation of Slug expression as an immediate reaction that probably triggers off a signaling cascade leading to Snail expression.

CONCLUSIONS

These results indicate distinct roles of the transcriptional factors Snail and Slug during ovarian cancer metastasis and cell survival through mediation of EMT.

Wnt-dependent beta-catenin signaling is activated after unilateral ureteral obstruction, and recombinant secreted frizzled-related protein 4 alters the progression of renal fibrosis

beta-Catenin functions as a transducer of Wnt signals to the nucleus, where it interacts with the T cell factor (TCF) family of DNA binding proteins to regulate gene expression. On the basis of the genes regulated by beta-catenin and TCF in various biologic settings, two predicted functions of beta-catenin/TCF-dependent transcription are to mediate the loss of epithelial polarity and to promote fibroblast activities, such as the increased synthesis of fibronectin during chronic renal disease. These predictions were tested by determination of the expression and function of an inhibitor of Wnt signaling, secreted frizzled-related protein 4 (sFRP4), during renal tubular epithelial injury initiated by unilateral ureteral obstruction (UUO). Despite increased sFRP4 gene expression in perivascular regions of injured kidneys, total sFRP4 protein levels decreased after injury. The decreased sFRP4 protein levels after UUO accompanied increased Wnt-dependent beta-catenin signaling in tubular epithelial and interstitial cells, along with increased expression of markers of fibrosis. Administration of recombinant sFRP4 protein caused a reduction in tubular epithelial beta-catenin signaling and suppressed the progression of renal fibrosis, as evidenced by a partial maintenance of E-cadherin mRNA expression and a reduction in the amount of fibronectin and alpha-smooth muscle actin proteins. Furthermore, recombinant sFRP4 reduced the number of myofibroblasts, a central mediator of fibrosis. It is concluded that beta-catenin signaling is activated in tubular epithelial and interstitial cells after renal injury, and recombinant sFRP4 can interfere with epithelial de-differentiation and with fibroblast differentiation and function during progression of renal fibrosis.

Unraveling signalling cascades for the Snail family of transcription factors

During development and carcinogenesis, the gradient of different molecular factors, the availability of corresponding receptors and the interplay between different signalling cascades combine to orchestrate the different stages. A good understanding of both developmental processes and oncogenesis leads to new insights into normal and aberrant regulation, processes that share some mutual key players. In this review, we will focus on the Snail family of transcription factors. These proteins, which share an evolutionarily conserved role in invertebrates and vertebrates, are implicated in several developmental processes, but are involved in carcinogenesis as well. We will highlight the different signalling cascades leading to the expression of Snail and Slug and how these factors are regulated on the transcriptional level. Then we will focus on how these factors execute their functions by repression of the numerous target genes that have been described to date.

Zinc finger domain of Snail functions as a nuclear localization signal for importin β-mediated nuclear import pathway

Snail, a DNA-binding zinc finger protein, functions as a transcriptional repressor for genes including E-cadherin during development and the acquisition of tumor cell invasiveness. Human Snail is a 264-amino acid nuclear protein with an amino-terminal basic amino acid-rich domain (SNAG domain) and a carboxyl-terminal DNA-binding domain (zinc finger domain). A series of fusion proteins composed of green fluorescent protein (GFP) and portions of the Snail protein were generated, and their subcellular localization was examined. Fusion of the four zinc fingers to GFP led to the targeting of GFP to the nucleus, demonstrating that the zinc finger domain is sufficient for nuclear localization. Using an in vitro transport system, the nuclear import of Snail was reconstituted by importin (karyopherin) beta in the presence of Ran and NTF2. We further demonstrated that Snail binds directly to importin beta in a zinc finger domain-dependent manner. These results indicate that zinc finger domain of Snail functions as a nuclear localization signal and Snail can be transported into the nucleus in an importin beta-mediated manner.

Hypoxia/aglycemia alters expression of occludin and actin in brain endothelial cells

The blood-brain barrier (BBB) serves as a critical organ in the maintenance of central nervous system homeostasis and is disrupted in a number of neurological disorders, including stroke. We examined the effects of hypoxia/aglycemia on the expression and localization of tight junction proteins, and on the function of the BBB in an in vitro model system. A receptor-operated/store-operated calcium channel blocker, SKF 96365, was used to determine if calcium flux was important in mediating hypoxia/aglycemia effects on the BBB. Expression of the tight junction protein occludin increased after hypoxic/aglycemic stress when cells were exposed to SKF 96365; this was correlated with partial protection of membrane localization of occludin and inhibition of the hypoxia-induced increase in permeability. Actin expression was dramatically reduced by hypoxia/aglycemia. Treatment with SKF 96365 during hypoxic stress protected monolayer permeability of sucrose, but transendothelial electrical resistances decreased with exposure to hypoxic stress regardless of treatment. Therefore, the presence of occludin at the membrane is dependent in part on calcium-sensitive signaling cascades; this provides a target for therapeutic intervention to minimize BBB disruption after stroke.

Emerging roles for p120-catenin in cell adhesion and cancer

Although originally identified as a Src substrate, p120-catenin (p120) is now known to regulate cell-cell adhesion through its interaction with the cytoplasmic tail of classical and type II cadherins. New evidence indicates that p120 regulates cadherin turnover at the cell surface, thereby controlling the amount of cadherin available for cell-cell adhesion. This function is necessary but not sufficient to promote strong adhesion, which is further controlled by signals acting on the amino-terminal p120 regulatory domain. p120 also modulates the activities of RhoA, Rac, and Cdc42, suggesting that along with other Src substrates, p120 regulates actin dynamics. Thus, p120 is a master regulator of cadherin abundance and activity, and likely participates in regulating the balance between adhesive and motile cellular phenotypes. This review summarizes recent progress in understanding mechanisms of p120 action, and discusses new implications with respect to roles for p120 in disease and cancer.

Barriers built on claudins

The fundamental functions of epithelia and endothelia in multicellular organisms are to separate compositionally distinct compartments and regulate the exchange of small solutes and other substances between them. Tight junctions (TJs) between adjacent cells constitute the barrier to the passage of ions and molecules through the paracellular pathway and function as a 'fence' within the plasma membrane to create and maintain apical and basolateral membrane domains. How TJs achieve this is only beginning to be understood. Recently identified components of TJs include the claudins, a family of four-transmembrane-span proteins that are prime candidates for molecules that function in TJ permeability. Their identification and characterization have provided new insight into the diversity of different TJs and heterogeneity of barrier functions in different epithelia and endothelia.

HGF converts ErbB2/Neu epithelial morphogenesis to cell invasion

Activation of the hepatocyte growth factor receptor Met induces a morphogenic response and stimulates the formation of branching tubules by Madin-Darby canine kidney (MDCK) epithelial cells in three-dimensional cultures. A constitutively activated ErbB2/Neu receptor, NeuNT, promotes a similar invasive morphogenic program in MDCK cells. Because both receptors are expressed in breast epithelia, are associated with poor prognosis, and hepatocyte growth factor (HGF) is expressed in stroma, we examined the consequence of cooperation between these signals. We show that HGF disrupts NeuNT-induced epithelial morphogenesis, stimulating the breakdown of cell-cell junctions, dispersal, and invasion of single cells. This correlates with a decrease in junctional proteins claudin-1 and E-cadherin, in addition to the internalization of the tight junction protein ZO-1. HGF-induced invasion of NT-expressing cells is abrogated by pretreatment with a pharmacological inhibitor of the mitogen-activated protein kinase kinase (MEK) pathway, which restores E-cadherin and ZO-1 at cell-cell junctions, establishing the involvement of MEK-dependent pathways in this process. These results demonstrate that physiological signals downstream from the HGF/Met receptor synergize with ErbB2/Neu to enhance the malignant phenotype, promoting the breakdown of cell-cell junctions and enhanced cell invasion. This is particularly important for cancers where ErbB2/Neu is overexpressed and HGF is a physiological growth factor found in the stroma.