Src and Fyn define a new signaling cascade activated by canonical and non-canonical Wnt ligands and required for gene transcription and cell invasion

Wnt ligands signal through canonical or non-canonical signaling pathways. Although both routes share common elements, such as the Fz2 receptor, they differ in the co-receptor and in many of the final responses; for instance, whereas canonical Wnts increase β-catenin stability, non-canonical ligands downregulate it. However, both types of ligands stimulate tumor cell invasion. We show here that both the canonical Wnt3a and the non-canonical Wnt5a stimulate Fz2 tyrosine phosphorylation, Fyn binding to Fz2, Fyn activation and Fyn-dependent Stat3 phosphorylation. Wnt3a and Wnt5a require Src for Fz2 tyrosine phosphorylation; Src binds to canonical and non-canonical co-receptors (LRP5/6 and Ror2, respectively) and is activated by Wnt3a and Wnt5a. This Fz2/Fyn/Stat3 branch is incompatible with the classical Fz2/Dvl2 pathway as shown by experiments of over-expression or depletion. Fyn is necessary for transcription of genes associated with invasiveness, such as Snail1, and for activation of cell invasion by both Wnt ligands. Our results extend the knowledge about canonical Wnt pathways, demonstrating additional roles for Fyn in this pathway and describing how this protein kinase is activated by both canonical and non-canonical Wnts.

Tubular Cytoplasmic Expression of Zinc Finger Protein SNAI1 in Renal Transplant Biopsies: A Sign of Diseased Epithelial Phenotype?

The aim of the present study was to analyze in vivo the role of zinc finger protein SNAI1 (SNAI1) on renal fibrosis. Unilateral ureteral obstruction injury was induced in Snai1 knockout mice. Snai1 gene deletion was, however, only partial and could therefore not be correlated to reduced fibrosis. Expression of SNAI1 protein and epithelial-mesenchymal transformation markers was then assessed in human chronic allograft nephropathy biopsy specimens. Significant up-regulation of SNAI1 protein was detected within cytoplasm of proximal tubules localized, for some of them, near foci of fibrosis and tubular atrophy. No concomitant epithelial-mesenchymal transformation could, however, be demonstrated analyzing the expression of the fibroblast markers vimentin, α-smooth muscle actin, and S100A4. SNAI1 cytoplasmic up-regulation was particularly evident in biopsy specimens obtained from calcineurin inhibitor-treated patients, which might be because of, as suggested by in vitro experiments, a decrease of the proteasome chimotrypsin activity. Deeper analysis on chronic allograft nephropathy biopsy specimens suggested that SNAI1 cytoplasmic up-regulation was preceded by a transient increase of phosphorylated heat shock protein 27, p38 mitogen-activated protein kinase, and glycogen synthase kinase 3β. Concomitant down-regulation of the polyubuquitinylated conjugates was detected in SNAI1⁺ tubules. Altogether, these results might suggest that calcineurin inhibitor-induced tubular SNAI1 protein cytoplasmic accumulation, possibly because of impaired SNAI1 proteasomal degradation and nuclear translocation, might be a sign of a diseased profibrotic epithelial phenotype.

FGFR4 role in epithelial-mesenchymal transition and its therapeutic value in colorectal cancer

Fibroblast growth factor receptor 4 (FGFR4) is vital in early development and tissue repair. FGFR4 expression levels are very restricted in adult tissues, except in several solid tumors including colorectal cancer, which showed overexpression of FGFR4. Here, FGFR4 mutation analysis discarded the presence of activating mutations, other than Arg(388), in different colorectal cancer cell lines and tumoral samples. Stable shRNA FGFR4-silencing in SW480 and SW48 cell lines resulted in a significant decrease in cell proliferation, adhesion, cell migration and invasion. This decrease in the tumorigenic and invasive capabilities of colorectal cancer cells was accompanied by a decrease of Snail, Twist and TGFβ gene expression levels and an increase of E-cadherin, causing a reversion to a more epithelial phenotype, in three different cell lines. In addition, FGFR4-signaling activated the oncogenic SRC, ERK1/2 and AKT pathways in colon cancer cells and promoted an increase in cell survival. The relevance of FGFR4 in tumor growth was supported by two different strategies. Kinase inhibitors abrogated FGFR4-related cell growth and signaling pathways at the same extent than FGFR4-silenced cells. Specific FGFR4-targeting using antibodies provoked a similar reduction in cell growth. Moreover, FGFR4 knock-down cells displayed a reduced capacity for in vivo tumor formation and angiogenesis in nude mice. Collectively, our data support a crucial role for FGFR4 in tumorigenesis, invasion and survival in colorectal cancer. In addition, FGFR4 targeting demonstrated its applicability for colorectal cancer therapy.

Nuclear Snail1 and nuclear ZEB1 protein expression in invasive and intraductal human breast carcinomas

Snail1 and ZEB1 are transcriptional repressors that drive tumor initiation and metastasis in animal models. Snail1 and ZEB1 are frequently coexpressed in tumor cell lines, suggesting that these factors may cooperate to promote tumor progression. However, coexpression of these transcriptional repressors in primary human cancer specimens has not been investigated. Previous studies assessed expression in primary breast cancers of Snail1 messenger RNA, which does not reflect Snail1 activity because Snail1 is subject to posttranslational modifications that inhibit its nuclear localization/activity. In the current study, using breast tumor cell lines of known Snail1 and ZEB1 expression status, we developed immunohistochemistry protocols for detecting nuclear Snail1 and nuclear ZEB1 proteins. Using these protocols, we assessed nuclear Snail1 and nuclear ZEB1 expressions in primary human breast cancers of varying subtypes (n = 78). Nuclear Snail1 and estrogen receptor α expressions were inversely associated in primary breast cancers, and nuclear Snail1 was expressed in approximately 80% of triple-negative breast cancers (lacking estrogen receptor α, progesterone receptor, and human epidermal growth factor receptor 2 overexpression). In contrast, nuclear ZEB1 was expressed at a significantly lower frequency in these breast cancers. Notably, nuclear Snail1 protein was detected in 45% of ductal carcinoma in situ specimens (n = 29), raising the important possibility that nuclear Snail1 expression in early stage breast lesions may predict future development of invasive breast cancer. Collectively, our studies demonstrate frequent expression of nuclear Snail1, but not nuclear ZEB1, in invasive, triple-negative breast cancers as well as in intraductal carcinomas.

Transcriptional crosstalk between TGF-β and stem cell pathways in tumor cell invasion: role of EMT promoting Smad complexes

Tumor cells undergoing the epithelial-mesenchymal transition (EMT) acquire the capacity to migrate, invade the stroma and metastasize. EMT cells also acquire stem cell characteristics suggesting crosstalk between EMT and stem cell pathways and contribution of the EMT process to the generation of cancer stem cells. Indeed, transforming growth factor-beta (TGF-β), a major inducer of EMT, cooperates with stem cell pathways like Wnt, Ras, Hedgehog and Notch to induce EMT. A molecular basis for this cooperative signaling is indicated by recent data showing that many EMT associated transcription factors like Snail1, Zeb1/2, Twist, β-catenin, Lef/TCF, Foxc2 and AP-1 interact with Smads and form EMT promoting Smad complexes (EPSC) engaged in both repressing epithelial genes and activating mesenchymal genes. Thus, formation and activation of EPSC seems to represent a point of convergence between EMT and stem cell pathways. Here, we review our current understanding of the mechanisms involved in the transcriptional crosstalk between TGF-β and stem cell pathways and discuss how a fundament for the activation of these mechanisms may lead to the induction of EMT in tumors.

Snail family regulation and epithelial mesenchymal transitions in breast cancer progression

Since its initial description, the interconversion between epithelial and mesenchymal cells (designed as epithelial-mesenchymal or mesenchymal-epithelial transition, EMT or MET, respectively) has received special attention since it provides epithelial cells with migratory features. Different studies using cell lines have identified cytokines, intercellular signaling elements and transcriptional factors capable of regulating this process. Particularly, the identification of Snail family members as key effectors of EMT has opened new ways for the study of this cellular process. In this article we discuss the molecular pathways that control EMT, showing a very tight and interdependent regulation. We also analyze the contribution of EMT and Snail genes in the process of tumorigenesis using the mammary gland as cellular model.

A SNAIL1-SMAD3/4 transcriptional repressor complex promotes TGF-beta mediated epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is essential for organogenesis and is triggered during carcinoma progression to an invasive state. Transforming growth factor-beta (TGF-beta) cooperates with signalling pathways, such as Ras and Wnt, to induce EMT, but the molecular mechanisms are not clear. Here, we report that SMAD3 and SMAD4 interact and form a complex with SNAIL1, a transcriptional repressor and promoter of EMT. The SNAIL1-SMAD3/4 complex was targeted to the gene promoters of CAR, a tight-junction protein, and E-cadherin during TGF-beta-driven EMT in breast epithelial cells. SNAIL1 and SMAD3/4 acted as co-repressors of CAR, occludin, claudin-3 and E-cadherin promoters in transfected cells. Conversely, co-silencing of SNAIL1 and SMAD4 by siRNA inhibited repression of CAR and occludin during EMT. Moreover, loss of CAR and E-cadherin correlated with nuclear co-expression of SNAIL1 and SMAD3/4 in a mouse model of breast carcinoma and at the invasive fronts of human breast cancer. We propose that activation of a SNAIL1-SMAD3/4 transcriptional complex represents a mechanism of gene repression during EMT.

Mesenchymal cells reactivate Snail1 expression to drive three-dimensional invasion programs

Epithelial–mesenchymal transition (EMT) is required for mesodermal differentiation during development. The zinc-finger transcription factor, Snail1, can trigger EMT and is sufficient to transcriptionally reprogram epithelial cells toward a mesenchymal phenotype during neoplasia and fibrosis. Whether Snail1 also regulates the behavior of terminally differentiated mesenchymal cells remains unexplored. Using a Snai1 conditional knockout model, we now identify Snail1 as a regulator of normal mesenchymal cell function. Snail1 expression in normal fibroblasts can be induced by agonists known to promote proliferation and invasion in vivo. When challenged within a tissue-like, three-dimensional extracellular matrix, Snail1-deficient fibroblasts exhibit global alterations in gene expression, which include defects in membrane type-1 matrix metalloproteinase (MT1-MMP)-dependent invasive activity. Snail1-deficient fibroblasts explanted atop the live chick chorioallantoic membrane lack tissue-invasive potential and fail to induce angiogenesis. These findings establish key functions for the EMT regulator Snail1 after terminal differentiation of mesenchymal cells.

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.

Endothelin-1 promotes epithelial-to-mesenchymal transition in human ovarian cancer cells

Despite considerable efforts to improve early detection and advances in chemotherapy, metastatic relapses remain a major challenge in the management of ovarian cancer. The endothelin A receptor (ET(A)R)/endothelin-1 (ET-1) axis has been shown to have a significant role in ovarian carcinoma by promoting tumorigenesis. Here we show that the ET-1/ET(A)R autocrine pathway drives epithelial-to-mesenchymal transition (EMT) in ovarian tumor cells by inducing a fibroblastoid and invasive phenotype, down-regulation of E-cadherin, increased levels of beta-catenin, Snail, and other mesenchymal markers, and suppression of E-cadherin promoter activity. Activation of ET(A)R by ET-1 triggers an integrin-linked kinase (ILK)-mediated signaling pathway leading to glycogen synthase kinase-3beta (GSK-3beta) inhibition, Snail and beta-catenin stabilization, and regulation of transcriptional programs that control EMT. Transfection of dominant negative ILK or exposure to an ILK inhibitor suppresses the ET-1-induced phosphorylation of GSK-3beta as well as Snail and beta-catenin protein stability, activity, and invasiveness, indicating that ET-1/ET(A)R-induced EMT-promoting effects depend on ILK. ET(A)R blockade by specific antagonists or reduction by ET(A)R RNA interference reverses EMT and cell invasion by inhibiting autocrine signaling pathways. In ovarian carcinoma xenografts, ABT-627, a specific ET(A)R antagonist, suppresses EMT determinants and tumor growth. In human ovarian cancers, ET(A)R expression is associated with E-cadherin down-regulation, N-cadherin expression, and tumor grade. Collectively, these findings provide evidence of a critical role for the ET-1/ET(A)R axis during distinct steps of ovarian carcinoma progression and identify novel targets of therapeutic intervention.