E-cadherin's dark side: Possible role in tumor progression

In the context of cancer, E-cadherin has traditionally been categorized as a tumor suppressor, given its essential role in the formation of proper intercellular junctions, and its downregulation in the process of epithelial-mesenchymal transition (EMT) in epithelial tumor progression. Germline or somatic mutations in the E-cadherin gene (CDH1) or downregulation by epigenetic mechanisms have been described in a small subset of epithelial cancers. However, recent evidence also points toward a promoting role of E-cadherin in several aspects of tumor progression. This includes preserved (or increased) E-cadherin expression in microemboli of inflammatory breast carcinoma, a possible "mesenchymal to epithelial transition" (MET) in ovarian carcinoma, collective cell invasion in some epithelial cancers, a recent association of E-cadherin expression with a more aggressive brain tumor subset, as well as the intriguing possibility of E-cadherin involvement in specific signaling networks in the cytoplasm and/or nucleus. In this review we address a lesser-known, positive role for E-cadherin in cancer.     

E-cadherin's dark side: Possible role in tumor progression

In the context of cancer, E-cadherin has traditionally been categorized as a tumor suppressor, given its essential role in the formation of proper intercellular junctions, and its downregulation in the process of epithelial–mesenchymal transition (EMT) in epithelial tumor progression. Germline or somatic mutations in the E-cadherin gene (CDH1) or downregulation by epigenetic mechanisms have been described in a small subset of epithelial cancers. However, recent evidence also points toward a promoting role of E-cadherin in several aspects of tumor progression. This includes preserved (or increased) E-cadherin expression in microemboli of inflammatory breast carcinoma, a possible “mesenchymal to epithelial transition” (MET) in ovarian carcinoma, collective cell invasion in some epithelial cancers, a recent association of E-cadherin expression with a more aggressive brain tumor subset, as well as the intriguing possibility of E-cadherin involvement in specific signaling networks in the cytoplasm and/or nucleus. In this review we address a lesser-known, positive role for E-cadherin in cancer.     

E-cadherin is required for intestinal morphogenesis in the mouse

E-cadherin, the primary epithelial adherens junction protein, has been implicated as playing a critical role in nucleating formation of adherens junctions, tight junctions, and desmosomes. In addition to its role in maintaining structural tissue integrity, E-cadherin has also been suggested as an important modulator of cell signaling via interactions with its cytoplasmic binding partners, catenins, as well as with growth factor receptors. Therefore, we proposed that loss of E-cadherin from the developing mouse intestinal epithelium would disrupt intestinal epithelial morphogenesis and function. To test this hypothesis, we used a conditional knockout approach to eliminate E-cadherin specifically in the intestinal epithelium during embryonic development. We found that E-cadherin conditional knockout mice failed to survive, dying within the first 24 hours of birth. Examination of intestinal architecture at E18.5 demonstrated severe disruption to intestinal morphogenesis in animals lacking E-cadherin in the epithelium of the small intestine. We observed changes in epithelial cell shape as well as in the morphology of villi. Although junctional complexes were evident, junctions were abnormal, and barrier function was compromised in E-cadherin mutant intestine. We also identified changes in the epithelial cell populations present in E-cadherin conditional knockout animals. The number of proliferating cells was increased, whereas the number of enterocytes was decreased. Although Wnt/β-catenin target mRNAs were more abundant in mutants compared with controls, the amount of nuclear activated β-catenin protein was dramatically lower in mutants compared with controls. In summary, our data demonstrate that E-cadherin is essential for intestinal epithelial morphogenesis and homeostasis during embryonic development.     

E-钙粘素及相关蛋白的研究进展

E-钙粘素介导上皮细胞的同型相互作用,在形态发生、信号转导、细胞极性及组织细胞完整性的维持中起着重要作用。E-钙粘素介导的黏附功能紊乱将导致细胞间粘连松散,与肿瘤侵袭转移密切相关,与肾脏疾病的相关性也越来越受到重视。其相关蛋白在疾病的发生、发展过程中,也起了不可忽视的作用,现就E-钙粘素及相关蛋白的研究进展做一综述。     

E-钙粘素及相关蛋白的研究进展

E-钙粘素介导上皮细胞的同型相互作用,在形态发生、信号转导、细胞极性及组织细胞完整性的维持中起着重要作用。E-钙粘素介导的黏附功能紊乱将导致细胞间粘连松散,与肿瘤侵袭转移密切相关,与肾脏疾病的相关性也越来越受到重视。其相关蛋白在疾病的发生、发展过程中,也起了不可忽视的作用,现就E-钙粘素及相关蛋白的研究进展做一综述。     

Formation of E-cadherin-mediated cell-cell adhesion activates AKT and mitogen activated protein kinase via phosphatidylinositol 3 kinase and ligand-independent activation of epidermal growth factor receptor in ovarian cancer cells

E-cadherin is a well characterized adhesion molecule that plays a major role in epithelial cell adhesion. Based on findings that expression of E-cadherin is frequently lost in human epithelial cancers, it has been implicated as a tumor suppressor in carcinogenesis of most human epithelial cancers. However, in ovarian cancer development, our data from the current study showed that E-cadherin expression is uniquely elevated in 86.5% of benign, borderline, and malignant ovarian carcinomas irrespective of the degree of differentiation, whereas normal ovarian samples do not express E-cadherin. Thus, we hypothesize that E-cadherin may play a distinct role in the development of ovarian epithelial cancers. Using an E-cadherin-expressing ovarian cancer cell line OVCAR-3, we have demonstrated for the first time that the establishment of E-cadherin mediated cell-cell adhesions leads to the activation of Akt and MAPK. Akt activation is mediated through the activation of phosphatidylinositol 3 kinase, and both Akt and MAPK activation are mediated by an E-cadherin adhesion-induced ligand-independent activation of epidermal growth factor receptor. We have also demonstrated that suppression of E-cadherin function leads to retarded cell proliferation and reduced viability. We therefore suggest that the concurrent formation of E-cadherin adhesion and activation of downstream proliferation signals may enhance the proliferation and survival of ovarian cancer cells. Our data partly explain why E-cadherin is always expressed during ovarian tumor development and progression.     

E-cadherin is critical for collective sheet migration and is regulated by the chemokine CXCL12 protein during restitution

Chemokines and other immune mediators enhance epithelial barrier repair. The intestinal barrier is established by highly regulated cell-cell contacts between epithelial cells. The goal of these studies was to define the role for the chemokine CXCL12 in regulating E-cadherin during collective sheet migration during epithelial restitution. Mechanisms regulating E-cadherin were investigated using Caco2(BBE) and IEC-6 model epithelia. Genetic knockdown confirmed a critical role for E-cadherin in in vitro restitution and in vivo wound repair. During restitution, both CXCL12 and TGF-β1 tightened the monolayer by decreasing the paracellular space between migrating epithelial cells. However, CXCL12 differed from TGF-β1 by stimulating the significant increase in E-cadherin membrane localization during restitution. Chemokine-stimulated relocalization of E-cadherin was paralleled by an increase in barrier integrity of polarized epithelium during restitution. CXCL12 activation of its cognate receptor CXCR4 stimulated E-cadherin localization and monolayer tightening through Rho-associated protein kinase activation and F-actin reorganization. These data demonstrate a key role for E-cadherin in intestinal epithelial restitution.     

Distinct roles of cadherin-6 and E-cadherin in tubulogenesis and lumen formation

Classic cadherins are important regulators of tissue morphogenesis. The predominant cadherin in epithelial cells, E-cadherin, has been extensively studied because of its critical role in normal epithelial development and carcinogenesis. Epithelial cells may also coexpress other cadherins, but their roles are less clear. The Madin Darby canine kidney (MDCK) cell line has been a popular mammalian model to investigate the role of E-cadherin in epithelial polarization and tubulogenesis. However, MDCK cells also express relatively high levels of cadherin-6, and it is unclear whether the functions of this cadherin are redundant to those of E-cadherin. We investigate the specific roles of both cadherins using a knockdown approach. Although we find that both cadherins are able to form adherens junctions at the basolateral surface, we show that they have specific and mutually exclusive roles in epithelial morphogenesis. Specifically, we find that cadherin-6 functions as an inhibitor of tubulogenesis, whereas E-cadherin is required for lumen formation. Ablation of cadherin-6 leads to the spontaneous formation of tubules, which depends on increased phosphoinositide 3-kinase (PI3K) activity. In contrast, loss of E-cadherin inhibits lumen formation by a mechanism independent of PI3K.     

CAS/CSE 1 stimulates E-cadhrin-dependent cell polarity in HT-29 human colon epithelial cells

The establishment and maintenance of epithelial polarity are crucial for tissue organization and function in mammals. Epithelial cadherin (E-cadherin) is expressed in epithelial cell membrane and is important for cell-cell adhesion, intercellular junctions formation, as well as epithelial cell polarization. We report herein that CAS (CAS/CSE 1), the human cellular apoptosis susceptibility protein, interacts with E-cadherin and stimulates polarization of HT-29 human colon epithelial cells. CAS binds with E-cadherin but not with beta-catenin in the immunoprecipitation assays. Interaction of CAS with E-cadherin enhances the formation of E-cadherin/beta-catenin cell-cell adhesive complex. Electron microscopic study demonstrated that CAS overexpression in cells stimulates intercellular junction complex formation. The disorganization of cellular cytoskeleton by cytochalasin D, colchicine, or acrylamide treatment disrupts CAS-stimulated HT-29 cell polarization. CAS-mediated HT-29 cell polarity is also inhibited by antisense E-cadherin DNA expression. Our results indicate that CAS cooperates with E-cadherin and plays a role in the establishment of epithelial cell polarity.     

CA125/MUC16 interacts with Src family kinases,and over-expression of its C-terminal fragment in human epithelial cancer cells reduces cell–cell adhesion

MUC16/CA125 is over-expressed in human epithelial tumors including ovarian, breast and some other carcinomas. The purpose of this study is to investigate how cell surface MUC16 is functionally involved in tumor progression, with a special focus on the role of its cytoplasmic tail. Forced expression of C-terminal MUC16 fragment (MUC16C) in epithelial cancer cells increased cell migration. We found that MUC16C directly interacted with Src family kinases (SFKs). Notably, localizations of E-cadherin and β-catenin at the cell–cell contacts were more diffuse in MUC16C transfectants compared with mock transfectants. Furthermore, MUC16C transfectants showed reduced Ca²⁺-dependent cell–cell adhesion, but the treatment of cells with PP2, a SFKs inhibitor, restored this. Because cell surface MUC16 is also associated with the E-cadherin/β-catenin complex, the over-expression of MUC16 and its interaction with SFKs may enhance SFKs-induced deregulation of E-cadherin. Thus, our results suggest a role for cell surface MUC16 in cell–cell adhesion of epithelial cancer cells.     

A novel role for p120 catenin in E-cadherin function

Indirect evidence suggests that p120-catenin (p120) can both positively and negatively affect cadherin adhesiveness. Here we show that the p120 gene is mutated in SW48 cells, and that the cadherin adhesion system is impaired as a direct consequence of p120 insufficiency. Restoring normal levels of p120 caused a striking reversion from poorly differentiated to cobblestone-like epithelial morphology, indicating a crucial role for p120 in reactivation of E-cadherin function. The rescue efficiency was enhanced by increased levels of p120, and reduced by the presence of the phosphorylation domain, a region previously postulated to confer negative regulation. Surprisingly, the rescue was associated with substantially increased levels of E-cadherin. E-cadherin mRNA levels were unaffected by p120 expression, but E-cadherin half-life was more than doubled. Direct p120-E-cadherin interaction was crucial, as p120 deletion analysis revealed a perfect correlation between E-cadherin binding and rescue of epithelial morphology. Interestingly, the epithelial morphology could also be rescued by forced expression of either WT E-cadherin or a p120-uncoupled mutant. Thus, the effects of uncoupling p120 from E-cadherin can be at least partially overcome by artificially maintaining high levels of cadherin expression. These data reveal a cooperative interaction between p120 and E-cadherin and a novel role for p120 that is likely indispensable in normal cells.     

A Novel Role for p115RhoGEF in Regulation of Epithelial Plasticity

Epithelial plasticity plays a critical role during physiological processes, such as wound healing and tissue regeneration, and dysregulation of epithelial plasticity can lead to pathological conditions, such as cancer. Cell-cell junctions are a critical feature of epithelial cells and loss of junctions is associated with acquisition of mesenchymal features, such as enhanced protrusion and migration. Although Rho has been implicated in regulation of junctions in epithelial cells, the role of Rho signaling in the regulation of epithelial plasticity has not been understood. We show that members of the RGS RhoGEFs family play a critical role in regulation of epithelial cell-cell junctions in breast epithelial cells. We identify a novel role for p115RhoGEF in regulation of epithelial plasticity. Loss of p115RhoGEF leads to decreased junctional E-cadherin and enhanced protrusiveness and migration. Conversely, overexpression of p115RhoGEF enhanced junctional E-cadherin and inhibited cell protrusion and migration. siRNA screen of 23 Rho effectors showed that members of the Diaphanous-Related Formin (DRF) family are required for p115RhoGEF-mediated changes in epithelial plasticity. Thus, our data indicates a novel role for p115RhoGEF in regulation of epithelial plasticity, which is dependent on Rho-DRF signaling module.     

Regulation of adherens junctions and epithelial paracellular permeability: a novel function for polyamines

Maintenance of intestinal mucosal epithelial integrity requires polyamines that are involved in the multiple signaling pathways controlling gene expression and different epithelial cell functions. Integrity of the intestinal epithelial barrier depends on a complex of proteins composing different intercellular junctions, including tight junctions, adherens junctions, and desmosomes. E-cadherin is primarily found at the adherens junctions and plays a critical role in cell-cell adhesions that are fundamental to formation of the intestinal epithelial barrier. The current study determined whether polyamines regulate intestinal epithelial barrier function by altering E-cadherin expression. Depletion of cellular polyamines by alpha-difluoromethylornithine (DFMO) reduced intracellular free Ca2+ concentration ([Ca2+]cyt), decreased E-cadherin expression, and increased paracellular permeability in normal intestinal epithelial cells (IEC-6 line). Polyamine depletion did not alter expression of tight junction proteins such as zona occludens (ZO)-1, ZO-2, and junctional adhesion molecule (JAM)-1. Addition of exogenous polyamine spermidine reversed the effects of DFMO on [Ca2+]cyt and E-cadherin expression and restored paracellular permeability to near normal. Elevation of [Ca2+]cyt by the Ca2+ ionophore ionomycin increased E-cadherin expression in polyamine-deficient cells. In contrast, reduction of [Ca2+]cyt by polyamine depletion or removal of extracellular Ca2+ not only inhibited expression of E-cadherin mRNA but also decreased the half-life of E-cadherin protein. These results indicate that polyamines regulate intestinal epithelial paracellular barrier function by altering E-cadherin expression and that polyamines are essential for E-cadherin expression at least partially through [Ca2+]cyt.     

DDR1 triggers epithelial cell differentiation by promoting cell adhesion through stabilization of E-cadherin

Discoidin domain receptor 1 (DDR1) promotes E-cadherin-mediated adhesion. The underlying mechanism and its significance, however, have not been elucidated. Here we show that DDR1 overexpression augmented, whereas dominant negative mutant (DN-DDR1) or knockdown of DDR1 inhibited E-cadherin localized in cell-cell junctions in epithelial cells. DDR1 changed the localization and abundance of E-cadherin, as well as epithelial plasticity, as manifested by enhancement of microvilli formation and alteration of cytoskeletal organization. DDR1 also reduced protein abundance of mesenchymal markers, whereas DN-DDR1 and sh-DDR1 showed opposite effects. These results suggest that expression of DDR1 increases epithelial plasticity. Expression of DDR1 augmented E-cadherin protein levels by decreasing its degradation rate. Photobleaching and photoconversion of E-cadherin conjugated with Eos fluorescence protein demonstrated that DDR1 increased the stability of E-cadherin on the cell membrane, whereas sh-DDR1 decreased it. Pull-down assay and expression of constitutively active or dominant-negative Cdc42 showed that DDR1 stabilized E-cadherin through inactivation of Cdc42. Altogether, our results show that DDR1 promotes cell-cell adhesion and differentiation through stabilization of E-cadherin, which is mediated by Cdc42 inactivation.