Coendocytosis of cadherin and c-Met coupled to disruption of cell-cell adhesion in MDCK cells--regulation by Rho, Rac and Rab small G proteins

Regulation of cadherin trafficking

Cadherins are a large family of cell-cell adhesion molecules that tether cytoskeletal networks of actin and intermediate filaments to the plasma membrane. This function of cadherins promotes tissue organization and integrity, as demonstrated by numerous disease states that are characterized by the loss of cadherin-based adhesion. However, plasticity in cell adhesion is often required in cellular processes such as tissue patterning during development and epithelial migration during wound healing. Recent work has revealed a pivotal role for various membrane trafficking pathways in regulating cellular transitions between quiescent adhesive states and more dynamic phenotypes. The regulation of cadherins by membrane trafficking is emerging as a key player in this balancing act, and studies are beginning to reveal how this process goes awry in the context of disease. This review summarizes the current understanding of how cadherins are routed and how the interface between cadherins and membrane trafficking pathways regulates cell surface adhesive potential. Particular emphasis is placed on the regulation of cadherin trafficking by catenins and the interplay between growth factor signaling pathways and cadherin endocytosis.

Roles of cell adhesion molecules nectin and nectin-like molecule-5 in the regulation of cell movement and proliferation

In response to chemoattractants, migrating cells form protrusions, such as lamellipodia and filopodia, and structures, such as ruffles over lamellipodia, focal complexes and focal adhesions at leading edges. The formation of these leading edge structures is essential for directional cell movement. Nectin-like molecule-5 (Necl-5) interacts in cis with PDGF receptor and integrin alpha(v)beta(3), and enhances the activation of signalling molecules associated with these transmembrane proteins, which results in the formation of leading edge structures and enhancement of directional cell movement. When migrating cells come into contact with each other, cell-cell adhesion is initiated, resulting in reduced cell velocity. Necl-5 first interacts in trans with nectin-3. This interaction is transient and induces down-regulation of Necl-5 expression at the cell surface, resulting in reduced cell movement. Cell proliferation is also suppressed by the down-regulation of Necl-5, because the inhibitory effect of Necl-5 on Sprouty2, a negative regulator of the Ras signalling, is diminished. PDGF receptor and integrin alpha(v)beta(3), which have interacted with Necl-5, then form a complex with nectin, which initiates cell-cell adhesion and recruits cadherin to the nectin-based cell-cell adhesion sites to form stable adherens junctions. The formation of adherens junctions stops cell movement, in part through inactivation of integrin alpha(v)beta(3) caused by the trans-interaction of nectin. Thus, nectin and Necl-5 play key roles in the regulation of cell movement and proliferation.

Changes in regulation of cell-cell adhesion during tumor transformation

Cadherin-mediated cell-cell adhesion defines the integrity of most tissues. Cell-cell adherens junctions are dynamic structures whose functional state is regulated by interactions of cadherin with beta-catenin, p120, and actin cytoskeleton structures. Small GTPases of the Rho family and GTPase Rap1 play the central role in the formation and maintenance of cell-cell adhesion. Aberrant activation of signaling pathways, transcriptional repression of the E-cadherin gene, ectopic expression of N-cadherin, and disturbances in regulation of adhesive and transcriptional functions of beta-catenin stimulate tumor progression.

The desmosome and pemphigus

Desmosomes are patch-like intercellular adhering junctions ("maculae adherentes"), which, in concert with the related adherens junctions, provide the mechanical strength to intercellular adhesion. Therefore, it is not surprising that desmosomes are abundant in tissues subjected to significant mechanical stress such as stratified epithelia and myocardium. Desmosomal adhesion is based on the Ca(2+)-dependent, homo- and heterophilic transinteraction of cadherin-type adhesion molecules. Desmosomal cadherins are anchored to the intermediate filament cytoskeleton by adaptor proteins of the armadillo and plakin families. Desmosomes are dynamic structures subjected to regulation and are therefore targets of signalling pathways, which control their molecular composition and adhesive properties. Moreover, evidence is emerging that desmosomal components themselves take part in outside-in signalling under physiologic and pathologic conditions. Disturbed desmosomal adhesion contributes to the pathogenesis of a number of diseases such as pemphigus, which is caused by autoantibodies against desmosomal cadherins. Beside pemphigus, desmosome-associated diseases are caused by other mechanisms such as genetic defects or bacterial toxins. Because most of these diseases affect the skin, desmosomes are interesting not only for cell biologists who are inspired by their complex structure and molecular composition, but also for clinical physicians who are confronted with patients suffering from severe blistering skin diseases such as pemphigus. To develop disease-specific therapeutic approaches, more insights into the molecular composition and regulation of desmosomes are required.

Regulation of epithelial cell adhesion and repulsion: role of endocytic recycling

A proper balance between cell adhesion and repulsion is essential for cellular morphogenesis during epithelial-mesenchymal transition and mesenchymal-epithelial transition. A number of ligand-receptor pairs including hepatocyte growth factor/scatter factor-Met and semaphorin-plexin are known to control this balance through the complex intracellular signaling pathways. Cell adhesion to other cells and extracellular matrix (ECM) is mediated by cell adhesion molecules (CAMs) and ECM receptors, respectively, which are associated with cytoskeleton through a variety of plaque proteins strengthening and/or weakening adhesion activities. Cell repulsion requires the downregulation of cell adhesion and the extensive changes in cytoskeletal dynamics. The endocytic recycling of CAMs and ECM receptors has recently emerged as an important mechanism to control the balance between cell adhesion and repulsion. Molecule interacting with CasL (MICAL) family proteins are originally identified as a plaque protein associated with ECM receptors integrins and implicated in semaphorin-plexin dependent repulsive axon guidance. We have recently shown that MICAL family protein JRAB/MICAL-L2 functions as an effector protein for Rab family small G protein Rab13 and regulates the endocytic recycling of tight junctional CAM occludin and controls the adhesion and repulsion of epithelial cells.

Cross-talk among integrin, cadherin, and growth factor receptor: roles of nectin and nectin-like molecule

Integrin, cadherin, and growth factor receptor are key molecules for fundamental cellular functions including cell movement, proliferation, differentiation, adhesion, and survival. These cell surface molecules cross-talk with each other in the regulation of such cellular functions. Nectin and nectin-like molecule (Necl) have been identified as cell adhesion molecules that belong to the immunoglobulin superfamily. Nectin and Necl play important roles in the integration of integrin, cadherin, and growth factor receptor at the cell-cell adhesion sites of contacting cells and at the leading edges of moving cells, and thus are also involved in the fundamental cellular functions together with integrin, cadherin, and growth factor receptor. This chapter describes how newly identified cell adhesion molecules, nectin and Necl, modulate the cross-talk among integrin, cadherin, and growth factor receptor and how these integrated molecules act in the regulation of fundamental cellular functions.

Mesenchymal transition in kidney collecting duct epithelial cells

Progressive organ damage due to tissue scarring and fibrosis is a paradigm shared by numerous human diseases including chronic kidney disease. The purpose of this study was to confirm the hypothesis that collecting duct (CD) epithelial cells can undergo mesenchymal transition (EMT) in vitro. The mechanism by which CDs undergo EMT is complex and involves both early and late cellular events. Early events include rapid insulin-like growth factor (IGF)-induced Akt and GSK-3beta phosphorylation, associated with early disruption of E-cadherin-beta-catenin membrane colocalization, with translocation of E-cadherin to endosomes, with translocation of beta-catenin to the nucleus, and with an increase in Snail expression. Transforming growth factor-beta1, on the other hand, induced early activation of Smad3 and its translocation to the nucleus, Erk1/2 phosphorylation, and early disruption of membrane E-cadherin localization. The late consequences of these events included a phenotypic transformation of the cells to a mesenchymal morphology with associated increase in vimentin and alpha-smooth muscle actin protein expression and a decrease in total cellular E-cadherin expression, detectable as early as 24 h after stimulation.

Making and breaking contacts: the cellular biology of cadherin regulation

Cadherin-mediated cell-cell interactions are dynamic processes, and cadherin function is tightly regulated in response to cellular context and signaling. Ultimately, cadherin regulation is likely to reflect the interplay between a range of fundamental cellular processes, including surface organization of receptors, cytoskeletal organization and cell trafficking, that are coordinated by signaling events. In this review we focus on recent advances in understanding how interplay with membrane trafficking and other cell-cell junctions can control cadherin function. The endocytosis of cadherins, and their post-internalization fate, influences surface expression and metabolic stability of these adhesion receptors. Similarly, at the surface, components of tight junctions provide a mode of cross-talk that regulates assembly of adherens junctions.

A cell-free assay for endocytosis of E-cadherin

E-Cadherin is a key cell-cell adhesion molecule at adherens junctions (AJs) and undergoes endocytosis when AJs are disrupted by the action of extracellular signals. To elucidate the mechanism of this endocytosis, we developed a new cell-free assay system for this reaction using the AJ-enriched fraction from rat liver. Non-trans-interacting E-cadherin, but not the trans-interacting one, underwent endocytosis in a clathrin-dependent manner. The endocytosis of trans-interacting E-cadherin was inhibited by Rac and Cdc42 small G proteins, which were activated by trans-interacting E-cadherin. Here, we describe the assay method for the endocytosis of E-cadherin from the AJ-enriched fraction prepared from rat liver.

Involvement of RhoA in progression of human hepatocellular carcinoma

BACKGROUND AND AIM

The activation of Rho proteins has been shown to lead to loss of polarity in cancer cells, as well as reorganization of the cytoskeleton and facilitation of cell motility, possibly resulting in their malignant potential. The clinicopathological significance of RhoA, however, is not yet well known in the case of hepatocellular carcinoma (HCC). This study evaluated the clinicopathological correlation of RhoA levels with HCC.

METHODS

The intratumor expression level of Rho was determined and compared with that in adjacent non-cancerous hepatic tissue using quantitative real time RT-PCR and Western blotting in 64 patients with HCC. Relationships between the level of RhoA and clinicopathological factors were examined. RhoA protein expression was confirmed by immunohistochemistry.

RESULTS

RhoA immunostaining was strong in malignant tissue whereas it was minimal in benign tissue. Tumor tissue of HCC patients demonstrated a copy number of RhoA mRNA that was well correlated with its protein level in each case and was significantly higher than that found in the corresponding non-cancerous liver tissue (P < 0.01). With regard to venous invasion, satellite lesions and advanced pTNM stage, the RhoA level tended to be higher in HCC than that seen in negative tissue (P < 0.05).

CONCLUSIONS

This is the first demonstration that the expression level of RhoA is correlated with tumor progression and metastasis in HCC. RhoA in tumor tissue might be expected to be not only a good candidate marker for invasive and proliferative tumor cells, but also a molecular target of these cells.

Lysophosphatidic acid modulates c-Met redistribution and hepatocyte growth factor/c-Met signaling in human bronchial epithelial cells through PKC delta and E-cadherin

Previously we demonstrated that ligation of lysophosphatidic acid (LPA) to G protein-coupled LPA receptors induces transactivation of receptor tyrosine kinases (RTKs), such as platelet-derived growth factor receptor beta (PDGF-Rbeta) and epidermal growth factor receptor (EGF-R), in primary cultures of human bronchial epithelial cells (HBEpCs). Here we examined the role of LPA on c-Met redistribution and modulation of hepatocyte growth factor (HGF)/c-Met pathways in HBEpCs. Treatment of HBEpCs with LPA-induced c-Met serine phosphorylation and redistribution to plasma membrane, while treatment with HGF-induced c-Met internalization. Pretreatment with LPA reversed HGF-induced c-Met internalization. Overexpression of dominant negative (Dn)-PKC delta or pretreatment with Rottlerin or Pertussis toxin (PTx) attenuated LPA-induced c-Met serine phosphorylation and redistribution. Co-immnuoprecipitation and immunocytochemistry showed that E-cadherin interacted with c-Met in HBEpCs. LPA treatment induced E-cadherin and c-Met complex redistribution to plasma membranes. Overexpression of Dn-PKC delta attenuated LPA-induced E-cadherin redistribution and E-cadherin siRNA attenuated LPA-induced c-Met redistribution to plasma membrane. Furthermore, pretreatment of LPA attenuated HGF-induced c-Met tyrosine phosphorylation and downstream signaling, such as Akt kinase phosphorylation and cell motility. These results demonstrate that LPA regulates c-Met function through PKC delta and E-cadherin in HBEpCs, suggesting an alternate function of the cross-talk between G-protein-coupled receptors (GPCRs) and RTKs in HBEpCs.

Sequential Down-regulation of E-Cadherin with Squamous Cell Carcinoma Progression: Loss of E-Cadherin via a Prostaglandin E2-EP2–Dependent Posttranslational Mechanism

The incidence of skin cancer is on the rise, with over 1 million new cases yearly. Although it is known that squamous cell cancers (SCC) are caused by UV light, the mechanism(s) involved remains poorly understood. In vitro studies with epithelial cells or reports examining malignant skin lesions suggest that loss of E-cadherin-mediated cell-cell contacts may contribute to SCCs. Other studies show a pivotal role for cyclooxygenase-dependent prostaglandin E2 (PGE2) synthesis in this process. Using chronically UV-irradiated SKH-1 mice, we show a sequential loss of E-cadherin-mediated cell-cell contacts as lesions progress from dysplasia to SCCs. This E-cadherin down-regulation was also evident after acute UV exposure in vivo. In both chronic and acute UV injury, E-cadherin levels declined at a time when epidermal PGE2 synthesis was enhanced. Inhibition of PGE2 synthesis by indomethacin in vitro, targeted deletion of EP2 in primary mouse keratinocyte (PMK) cultures or deletion of the EP2 receptor in vivo abrogated this UV-induced E-cadherin down-regulation. In contrast, addition of PGE2 or the EP2 receptor agonist butaprost to PMK produced a dose- and time-dependent decrease in E-cadherin. We also show that UV irradiation, via the PGE2-EP2 signaling pathway, may initiate tumorigenesis in keratinocytes by down-regulating E-cadherin-mediated cell-cell contacts through its mobilization away from the cell membrane, internalization into the cytoplasm, and shuttling through the lysosome and proteasome degradation pathways. Further understanding of how UV-PGE2-EP2 down-regulates E-cadherin may lead to novel chemopreventative strategies for the treatment of skin and other epithelial cancers.

CagA-independent disruption of adherence junction complexes involves E-cadherin shedding and implies multiple steps in Helicobacter pylori pathogenicity

Infection with Helicobacter pylori (H. pylori) leads to depolarization and migration of polarized epithelial cells, both strongly enhanced by injection of the pathogenic factor CagA (cytotoxin-associated gene A) into the host cytoplasm. Depolarization and migration of epithelial cells imply the disruption of cell adhesion junctions (AJs) comprising a protein complex of E-cadherin, beta-catenin, p120(ctn), and alpha-catenin. Here, we analyzed the disintegration of E-cadherin-mediated AJs and demonstrated that loss of E-cadherin-dependent cell-cell contacts is entirely independent of CagA. Upon infection with H. pylori, either wild-type (wt) or a cagA mutant (DeltacagA), interaction between E-cadherin and alpha-catenin dissociated rapidly, while binding of E-cadherin to beta-catenin and p120(ctn) was hardly affected. Simultaneously, loss of cell adhesion involved E-cadherin cleavage induced by a bacterial factor secreted by H. pylori. Finally, beta-catenin-mediated transcription, a hallmark of many carcinomas, was not activated in H. pylori-infected epithelial cells at this stage of infection. Altogether, our data indicate that H. pylori-induced pathogenesis is a multi-step process initiated by CagA-independent mechanisms. These include proteolytical cleavage of E-cadherin and dissociation of the E-cadherin/beta-catenin/p120(ctn) complex from the actin cytoskeleton by disrupting binding to alpha-catenin.

Blocking Wnt/LRP5 signaling by a soluble receptor modulates the epithelial to mesenchymal transition and suppresses met and metalloproteinases in osteosarcoma Saos-2 cells

We previously reported the Wnt receptor low-density lipoprotein receptor-related protein 5 (LRP5) was frequently expressed in osteosarcoma (OS) tissue and correlated with metastasis and a lower disease-free survival. Subsequent in vitro analysis revealed that dominant-negative, soluble LRP5 (sLRP5) can reduce in vitro cellular invasion. In the current study, we examined the molecular mechanisms of blocking canonical Wnt signaling by sLRP5 in Saos-2 osteosarcoma cells. Transfection of sLRP5 caused a marked up-regulation of E-cadherin in this cell line. This increase in E-cadherin, seen primarily at the cell-cell contact borders, was associated with down-regulation of Slug and Twist, transcriptional repressors which mediate cancer invasion and metastasis. In contrast, N-cadherin, a mesenchymal marker, was reduced by sLRP5. In addition, blocking Wnt signaling by sLRP5 modulated other epithelial and mesenchymal markers (keratin 8 and 18, fibronectin), suggesting a reversal of epithelial-mesenchymal transition (EMT) seen during cancer progression. SLRP5 also reduced the expression of matrix metalloproteinase (MMP) 2 and 14, consistent with a decrease in invasive capacity. SLRP5 transfection decreased both Met expression and hepatocyte growth factor (HGF)-induced cell motility. Taken together, these results support a role for Wnt/LRP5 signaling in invasiveness of a subset of OS cells.

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.

Increased Internalization of p120-uncoupled E-cadherin and a Requirement for a Dileucine Motif in the Cytoplasmic Domain for Endocytosis of the Protein

E-cadherin is a member of the cadherin family of Ca2+-dependent cell-cell adhesion molecules. E-cadherin associates with beta-catenin at the membrane-distal region of its cytosolic domain and with p120 at the membrane-proximal region of its cytoplasmic domain. It has been shown that a pool of cell surface E-cadherin is constitutively internalized and recycled back to the surface. Further, p120 knockdown by small interference RNA resulted in dose-dependent elimination of cell surface E-cadherin. Consistent with these observations, we found that selective uncoupling of p120 from E-cadherin by introduction of amino acid substitutions in the p120-binding site increased the level of E-cadherin endocytosis. The increased endocytosis was clathrin-dependent, because it was blocked by expression of a dominant-negative form of dynamin or by hypertonic shock. A dileucine motif in the juxtamembrane cytoplasmic domain is required for E-cadherin endocytosis, because substitution of these residues to alanine resulted in impaired internalization of the protein. The alanine substitutions in the p120-uncoupled construct reduced endocytosis of the protein, indicating that this motif was dominant to p120 binding in the control of E-cadherin endocytosis. Therefore, these results are consistent with the idea that p120 regulates E-cadherin endocytosis by masking the dileucine motif and preventing interactions with adaptor proteins required for internalization.

From Tpr-Met to Met, tumorigenesis and tubes

The receptor for hepatocyte growth factor (HGF)/scatter factor (SF), Met, controls a program of invasive epithelial growth through the coordination of cell proliferation and survival, cell migration and epithelial morphogenesis. This process is important during embryogenesis and for organ regeneration in the adult. However, when deregulated the HGF/SF-Met signaling axis contributes to tumorigenesis and metastasis. Studies on the oncogenic activation of the Met receptor have shed light on the molecular mechanisms underlying the oncogenic activation of receptor tyrosine kinase (RTKs). More than a decade ago, work on the Met related oncogene, Tpr-Met, revealed the mechanism for activation of RTK-derived oncogenes generated following chromosomal translocation. More recently, studies on the mechanisms of downregulation of the Met RTK highlight a role for loss of downregulation in RTK oncogenic activation.

Caveolin 2 regulates endocytosis and trafficking of the M1 muscarinic receptor in MDCK epithelial cells

Clathrin and caveolins are known for their involvement in the internalization of numerous receptors. Here we show that in polarized epithelial Madin-Darby canine kidney cells, both the clathrin machinery and caveolins are involved in the endocytosis and delivery to the plasma membrane (PM) of the M1 muscarinic acetylcholine receptor (mAChR). We initially localized this receptor to the lateral membrane, where it accumulates proximal to the tight junctions. From there it is internalized through the clathrin-mediated pathway. In addition, the receptor may associate on the PM with caveolin (cav) 2 or in intracellular compartments with either cav 2, or monomeric or oligomeric cav 1. Association of the PM M1 mAChR with cav 2 inhibits receptor endocytosis through the clathrin-mediated pathway or retains the receptor in an intracellular compartment. This intracellular association attenuates receptor trafficking. Expression of cav 1 with cav 2 rescues the latter's inhibitory effect. The caveolins stimulate M1 mAChR oligomerization thus maintaining a constant amount of monomeric receptor. These results provide evidence that caveolins play a role in the attenuation of the M1 muscarinic receptor's intracellular trafficking to and from the PM.

Cadherins in development: cell adhesion, sorting, and tissue morphogenesis

Tissue morphogenesis during development is dependent on activities of the cadherin family of cell-cell adhesion proteins that includes classical cadherins, protocadherins, and atypical cadherins (Fat, Dachsous, and Flamingo). The extracellular domain of cadherins contains characteristic repeats that regulate homophilic and heterophilic interactions during adhesion and cell sorting. Although cadherins may have originated to facilitate mechanical cell-cell adhesion, they have evolved to function in many other aspects of morphogenesis. These additional roles rely on cadherin interactions with a wide range of binding partners that modify their expression and adhesion activity by local regulation of the actin cytoskeleton and diverse signaling pathways. Here we examine how different members of the cadherin family act in different developmental contexts, and discuss the mechanisms involved.

Role of p120-catenin in cadherin trafficking

p120-catenin (p120) has emerged over the past several years as an important regulatory component of the cadherin adhesive complex. A core function of p120 in mammalian cells is to stabilize cadherins at the cell membrane by modulating cadherin membrane trafficking and degradation. In this way, p120 levels act as a set point mechanism that tunes cell-cell adhesive interactions. The primary control point for this regulatory activity appears to be at the level of cadherin internalization from the plasma membrane, although p120 may also impact other aspects of cadherin trafficking and turnover. In the following review, the general mechanisms of cadherin trafficking are discussed, and models for how p120 may influence cadherin membrane dynamics are presented. In one model, p120 may function as a "cap" to bind the cadherin cytoplasmic tail and prevent cadherin interactions with endocytic membrane trafficking machinery. Alternatively, p120 may stabilize cell junctions or regulate membrane trafficking machinery through interactions with small GTPases such as Rho A, Rac and Cdc42. Through these mechanisms p120 exerts influence over a wide range of biological processes that are dependent upon tight regulation of cell surface cadherin levels.

Coordinated functions of E-cadherin and transforming growth factor beta receptor II in vitro and in vivo

In epithelial cells, E-cadherin plays a key role in cell-cell adhesion, and loss of E-cadherin is a hallmark of tumor progression fostering cancer cell invasion and metastasis. To examine E-cadherin loss in squamous cell cancers, we used primary human esophageal epithelial cells (keratinocytes) as a platform and retrovirally transduced wild-type and dominant-negative forms of E-cadherin into these cells. We found decreased cell adhesion in the cells expressing dominant-negative E-cadherin, thereby resulting in enhanced migration and invasion. To analyze which molecular pathway(s) may modulate these changes, we conducted microarray analysis and found up-regulation of transforming growth factor beta receptor II (TbetaRII) in the wild-type E-cadherin-overexpressing cells, which was confirmed by real-time PCR and Western blot analyses. To investigate the in vivo relevance of this finding, we analyzed tissue microarrays of paired esophageal squamous cell carcinomas and adjacent normal esophagus, and we could show a coordinated loss of E-cadherin and TbetaRII in approximately 80% of tumors. To determine if there may be an E-cadherin-dependent regulation of TbetaRII, we show the physical interaction of E-cadherin with TbetaRII and that this is mediated through the extracellular domains of E-cadherin and TbetaRII, respectively. In addition, TbetaRI is recruited to this complex. When placed in the context of three-dimensional cell culture, which reflects the physiologic microenvironment, TbetaRII-mediated cell signaling is dependent upon intact E-cadherin function. Our results, which suggest that E-cadherin regulates TbetaRII function, have important implications for epithelial carcinogenesis characterized through the frequent occurrence of E-cadherin and TbetaRII loss.

Aberrant Wnt/beta-catenin signaling in pancreatic adenocarcinoma

Wnt/beta-catenin signaling plays an important role in normal development. However, its aberrant activation is associated with several cancers. The aim of this study is to examine the Wnt/beta-catenin pathway in patients with advanced pancreatic adenocarcinoma (n = 31). Paraffin sections from tumors (n = 16) and normal pancreata (n = 3) were used to determine the localization of beta-catenin. An additional 15 frozen tumors, adjacent normal pancreata (n = 5), or normal pancreata (n = 4) were utilized for protein isolation. Tumors were also examined for mutations in exon 3 of the CTNNB1 gene. More than 65% of the tumors showed an increase in total beta-catenin, consistent with its enhanced membranous, cytoplasmic, and nuclear localization, but only two showed mutations in CTNNB1. The majority of the remaining tumors demonstrated concurrent increases in Wnt-1 and frizzled-2 (positive regulators) and a decrease in Ser45/Thr41-phospho-beta-catenin. Electrophoretic mobility shift assay demonstrated beta-catenin-T-cell factor binding in tumors only. Adenomatous polyposis coli and axin, which are both negative regulators, remained unchanged. Unexpectedly, total glycogen synthase kinase-3beta protein was elevated in these tumors. Elevated levels of E-cadherin were also observed, although E-cadherin-beta-catenin association in tumors remained unaffected. Thus, Wnt/beta-catenin activation was observed in 65% of pancreatic adenocarcinomas, independently of beta-catenin gene mutations in most tumors.

CD47 regulation of epithelial cell spreading and migration, and its signal transduction

CD47 is an integrin-associated penta-transmembrane protein that possesses an immunoglobulin-like domain in its extracellular region. We have now investigated the role of CD47 in the regulation of epithelial cell spreading and migration. CD47 is colocalized with E-cadherin at cell-cell adhesion sites of epithelial cells. A Ca2+ switch experiment showed that CD47 was endocytosed and then relocalized to cell-cell adhesion sites in a similar manner to E-cadherin. Such polarized localization of CD47 required the multiple spanning region of this protein. Forced expression of CD47 induced cell spreading with marked lamellipodium formation and resulted in both partial disruption of cell-cell adhesion and enhancement of the hepatocyte growth factor-stimulated scattering of Madin-Darby canine kidney cells. The CD47-induced cell spreading was blocked by inhibition of Src and mitogen-activated protein kinase kinase. Thus, these results suggest that CD47 participates in the regulation of cell-cell adhesion and cell migration through reorganization of the actin cytoskeleton in epithelial cells. This function of CD47 is mediated by the activation of Src and mitogen-activated protein kinase kinase.

Involvement of a novel ADP-ribosylation factor GTPase-activating protein, SMAP, in membrane trafficking: implications in cancer cell biology

The endocytosis of cell membrane proteins is initiated by the binding of activated Arf6, a member of Ras-related GTPases, to the PM. A GAP specific for Arf6 triggers the budding of endocytotic vesicles from the PM by inactivating GTP-bound Arf6. We recently identified the SMAP gene that encodes an ArfGAP and is involved in the endocytosis of TfnR and possibly E-cadherin. In this review, we summarize the process of intracellular membrane trafficking, highlighting the roles played by the SMAP gene. Progression of cancer to malignancy occurs in parallel with the disappearance of E-cadherin, a central component of the adherens junction in epithelial cells. Therefore, elucidation of the molecular mechanism of E-cadherin endocytosis should be one of the key elements in tumor cell biology.

Induction of E-cadherin endocytosis by loss of protein phosphatase 2A expression in human breast cancers

The cell-cell adhesion molecule E-cadherin is stabilized by linking intracellularly with the actin cytoskeleton through PP2A-mediated recruitment of IQGAP1 to Rac1-bound E-cadherin-catenins complex in nonmalignant HME cells. However, little is known about the dysfunction of E-cadherin by loss or reduced expression of PP2A in human breast cancer cells. We report here that both human breast cancer MDA-MB-231 and MCF-7 cells were deficient in expression of the PP2A-A protein and lost the IQGAP1 recruitment to Rac1-bound catenins. In MDA-MB-231 cells, E-cadherin was also deficient. Immunohistochemical analysis of the normal-carcinoma matched human breast tissue arrays revealed that PP2A-A was expressed in 96% of normal tissue specimens but not in 57% of carcinoma specimens. Expression of E-cadherin in MCF-7 cells was 1.5-fold higher than that in HME cells, however, 80% of E-cadherin was endocytosed and incompletely anchored to F-actin. Therefore, we propose that the dysfunction of E-cadherin due to its endocytosis may occur in some proportion of human breast carcinomas in which the PP2A-A protein is lost or significantly reduced.

Ligand-of-Numb protein X is an endocytic scaffold for junctional adhesion molecule 4

Junctional adhesion molecule 4 (JAM4) is a cell adhesion molecule that interacts with a tight junction protein, membrane-associated guanylate kinase inverted 1 (MAGI-1). Our previous studies suggest that JAM4 is implicated in the regulation of paracellular permeability and the signalings of hepatocyte growth factor. In this study, we performed yeast two-hybrid screening to search for an unidentified JAM4-binding protein and obtained one isoform of Ligand-of-Numb protein X1 (LNX1), LNXp70, that is an interactor of Numb. Ligand-of-Numb protein X1 is expressed in kidney glomeruli and intestinal epithelial cells, where JAM4 is also detected. Immunoprecipitation from kidney lysates supports the in vivo interaction of proteins. Biochemical studies reveal that JAM4 directly binds the second PDZ domain of LNX1 through its carboxyl terminus. Junctional adhesion molecule 4, LNX1 and Numb form a tripartite complex in vitro and are partially colocalized in heterologous cells. Ligand-of-Numb protein X1 facilitates endocytosis of JAM4 and is involved in transforming growth factor beta -induced redistribution of JAM4 in mammary epithelial cells. Experiments using dominant-negative constructs and RNA interference insure that Numb is necessary for the LNX1-mediated endocytosis of JAM4. All these findings indicate that LNX1 provides an endocytic scaffold for JAM4 that is implicated in the reorganization of cell junctions.

Subtractive transcriptomics: establishing polarity drives in vitro human endothelial morphogenesis

Although investigations of mature normal and tumor-derived capillaries have resulted in characterization of these structures at the phenotypic level, less is known regarding the initial molecular cues for cellular assembly of endothelial cells into human capillaries. Here, we employ a novel combination of microenvironmental manipulation and microarray data filtration over narrowly delineated temporal data series to identify the morphogenesis component apart from the proliferation component, as pooled human microvascular-derived endothelial cells are induced to form capillary-like structures in vitro in a murine tumor-derived matrix. The 217 morphogenesis-specific genes identified using this subtractive transcriptomics approach are mostly independent of the angiogenic proteins currently used as therapeutic targets for aberrant angiogenesis. Quantitative real-time PCR was used to validate 20% of these transcripts. Immunofluorescent analysis of proliferating and tube-forming cells validates at the protein level the morphogenesis-specific expression pattern of 16 of the 217 gene products identified. The transcripts that are selectively up-regulated in tube-forming endothelial cells reveal a temporal expression pattern of genes primarily associated with intracellular trafficking, guided migration, cytoskeletal reorganization, cellular adhesion, and proliferation inhibition. These data show that a sequential up-regulation of genes that establish and maintain polarity occurs during migration and morphogenesis of in vitro human endothelial cells undergoing tubulogenesis; some of which may well be effective as novel antiangiogenic drug targets.

Raf plus TGFbeta-dependent EMT is initiated by endocytosis and lysosomal degradation of E-cadherin

Oncogenic Ras interferes with adhesive functions of epithelial cells, but requires tumor growth factor beta (TGFbeta) signaling to cause epithelial-mesenchymal transition (EMT) and tumor progression in model systems. To investigate the mechanisms by which Ras and TGFbeta pathways cooperate in EMT induction, we introduced a tamoxifen-inducible version of Raf-1 (RafER) into fully polarized, mammary epithelial cells (EpH4). EMT characterized by loss of E-cadherin expression and upregulation of invasiveness-promoting genes was induced by TGFbeta plus 4-hydroxytamoxifen (4HT) activation of RafER. Downregulation of E-cadherin by RafER plus TGFbeta was detectable in total cell lysates after 48 h and much earlier in detergent-insoluble fractions of E-cadherin. Both pathways cooperated to strongly enhance endocytosis of E-cadherin, mainly via the clathrin-dependent route. Pulse-chase experiments showed decreased E-cadherin protein stability in cells stimulated with TGFbeta and 4HT and increased E-cadherin half-life in the presence of monensin. Monensin and chloroquine prevented E-cadherin degradation to different extent, but only monensin effectively blocked the loss of E-cadherin from the junctional complexes. Both lysosome inhibitors caused accumulation of E-cadherin vesicles, some of which were positive for Cathepsin D and lysosome-associated membrane protein 1 (LAMP-1). In addition, TGFbeta and mitogen-activated protein kinase hyperactivation synergistically induced E-cadherin ubiquitination, suggesting that the cooperation of Raf and TGFbeta favors lysosomal degradation of E-cadherin instead of its recycling. Our data indicate that early stages of EMT involve cooperative, post-translational downregulation of E-cadherin, whereas loss of E-cadherin via transcriptional repression is a late event in EMT.

Coupling receptor tyrosine kinases to Rho GTPases--GEFs what's the link

Rho GTPases are molecular switches involved in the regulation of many cellular processes. This review summarizes work examining how stimulation of receptor tyrosine kinases (RTKs) leads to the activation of Rho guanine nucleotide exchange factors (GEFs) and their Rho GTPase substrates. The collective findings strongly suggest that RTK signaling to Rho proteins is a general signal transduction mechanism, like RTK mediated activation of phosphatidyl inositol 3-kinase, phospholipase Cgamma, and the mitogen activated protein kinase (MAPK) pathway. More than half of the 58 known human RTKs activate at least one Rho family member. Likewise, 16 Rho GEFs directly interact with and/or are phosphorylated by a RTK. The specificity of receptor tyrosine kinase/Rho GEF signaling seems to be somewhat promiscuous. There several cases where multiple RTKs activate the same Rho GEF and where a single RTK can activate multiple Rho GEFs. Expression analysis indicates that the average human tissue contains transcripts for 33 RTKs, 34 Rho GEFs, and 14 Rho GTPases with each tissue containing a unique complement of these proteins. Given the promiscuity of RTKs for Rho GEFs, Rho GEFs for Rho GTPases, and the large number of these proteins expressed in cells, a complex combinatorial network of proteins in these families may contribute to coding specific signals and cell responses from RTKs.

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.

Involvement of protein phosphatase 2A in the maintenance of E-cadherin-mediated cell-cell adhesion through recruitment of IQGAP1

Serine/threonine protein phosphatase (PP) 2A regulates many biological processes, however it remains unclear whether PP2A participates in cadherin-mediated cell-cell adhesion. We show here that the core enzyme of PP2A (PP2A-AC) is localized in the cell-cell adhesion sites between adjacent cells and associated with the E-cadherin-catenins complex in non-malignant human mammary epithelial (HME) cells at confluence. Treatment of the cells with either okadaic acid (OA), an inhibitor of PP2A, or siRNA for the regulatory subunit A of PP2A (PP2A-A) caused disruption of cell-cell adhesion and F-actin assembly, without affecting the complex formation of E-cadherin with beta- and alpha-catenins. While a small GTPase Rac and its effector IQGAP1 were associated with the E-cadherin-catenins complex, either OA or PP2A-A siRNA concomitantly induced the dissociation of IQGAP1, but not Rac, from the complex and the internalization of E-cadherin from the cell surface. We therefore propose that PP2A plays a crucial role in the maintenance of cell-cell adhesion through recruitment of IQGAP1 to the Rac-bound E-cadherin-catenins complex.

Mechanisms inactivating the gene for E-cadherin in sporadic gastric carcinomas

AIM: To study the role of CDH1/E-cadherin (E-cad) gene alteration profiles including mutation, loss of heterozygosity (LOH), promoter polymorphism and hypermethylation in mechanisms of CDH1 inactivation in gastric carcinoma (GC).

METHODS: Specimens were collected surgically from 70 patients with GC. Allelotyping PCR and detection of LOH, denaturing high pressure liquid chromatography and DNA sequencing, restriction fragment length polymorphism analysis, methylation specific PCR, and immunohistochemical staining were used.

RESULTS: Promoter polymorphism was not a major mechanism of E-cad inactivation. Only one truncating mutation was found in a diffuse type tumor (3%). Both LOH and promoter hypermethylation were major mechanisms of E-cad inactivation, but interestingly, there was a negative association between the fraction of allelic loss (LOH) in tumors and hypermethylation of CDH1. Therefore LOH and hypermethylation were two different tumorigenic pathways involved in GC.

CONCLUSION: Given the findings that somatic mutation was extremely low and the relationship between LOH and hypermethylation was inverse, any two combinations of these three factors cannot fulfill the classical two-hit hypothesis of CDH1 inactivation. Thus, other mechanisms operating at the transcriptional level or at the post-translational level might be required to induce E-cadherin inactivation.

Involvement of the Ras-Ras-activated Rab5 guanine nucleotide exchange factor RIN2-Rab5 pathway in the hepatocyte growth factor-induced endocytosis of E-cadherin

E-cadherin is a key cell-cell adhesion molecule at adherens junctions (AJs) and undergoes endocytosis when AJs are disrupted by the action of an extracellular signal, such as hepatocyte growth factor (HGF)/scatter factor. Rab5 small G protein has been implicated in the HGF-induced endocytosis of E-cadherin, but the molecular mechanism for the regulation of Rab5 activity remains unknown. We first studied this mechanism by using the cell-free assay system for the endocytosis of E-cadherin of the AJ-enriched fraction from rat livers. HGF induced activation of Ras small G protein, which then bound to RIN2, a Rab5 GDP/GTP exchange factor with the Vps9p-like guanine nucleotide exchange factor and Ras association domains, and activated it. Activated RIN2 then activated Rab5, eventually inducing the endocytosis of E-cadherin. We then studied whether RIN2 was involved in the HGF-induced endocytosis of E-cadherin in intact Madin-Darby canine kidney cells. RIN2 localized at the cell-cell adhesion sites, and its guanine nucleotide exchange factor activity was required for the HGF-induced endocytosis of E-cadherin in Madin-Darby canine kidney cells. These results indicate that RIN2 connects Ras to Rab5 in the HGF-induced endocytosis of E-cadherin.

Overexpression of ADAM9 enhances growth factor-mediated recycling of E-cadherin in human colon cancer cell line HT29 cells

Growth factor-mediated stimulation of epithelial cells induces the disassembly of E-cadherin-mediated cell-cell adhesion. We found that overexpression of a disintegrin and metalloprotease 9 (ADAM9) enhanced growth factor-mediated induction of endocytosis and dynamic recycling of E-cadherin in HT29 human colon cancer cells. In addition, ubiquitination and degradation of E-cadherin were reduced in these cells. ADAM9 constitutively interacted with E-cadherin, and the two proteins co-localized at the plasma membrane of HT29 cells. Administration of a metalloprotease inhibitor or overexpression of an ADAM9 mutant lacking metalloprotease activity attenuated growth factor-dependent endocytosis and recycling of E-cadherin as well as scattering of HT29 cells. These results suggest that the metalloprotease activity of ADAM9 mediates growth factor-induced endocytosis and dynamic recycling of E-cadherin and prevents E-cadherin degradation.

Rho family GTPases are activated during HGF-stimulated prostate cancer-cell scattering

An important process in embryogenesis and cancer-cell metastasis is the conversion of epithelial cells to a migratory phenotype, a phenomenon known as epithelial-mesenchymal transition (E-MT). To achieve E-MT, cells dissociate from neighbouring cells and adopt a migratory morphology. This transition requires remodelling of their cell shape and substratum adhesions; activities that require extensive reorganisation of the actin cytoskeleton. Hepatocyte growth factor (HGF)-induced scattering of Madin Darby canine kidney (MDCK) cells is a routinely used model of E-MT, in which actin cytoskeletal rearrangement is known to be dependent on Rho family GTPases. We have developed a novel model of HGF-induced E-MT using the human prostate cancer cell line, DU145. This model overcomes the limitation of using a canine cell line and facilitates the study of E-MT in human cancer. We demonstrate for the first time the scattering response of individual DU145 cells to HGF in real time and have characterised changes in actin cytoskeletal organisation and cell adhesions as these cells respond to HGF. HGF-induced scattering of DU145 cells is dependent on the activity of Rho family GTPases, and using this model, we are able to demonstrate for the first time that endogenous Cdc42 is activated downstream of HGF. Furthermore we have also shown that the response of DU145 cells to HGF is dependent on a phosphatidylinositide 3-kinase pathway.

Tumor suppressor in lung cancer (TSLC)1 suppresses epithelial cell scattering and tubulogenesis

The tumor suppressor in lung cancer 1 (TSLC1/IGSF4) encodes an immunoglobulin-superfamily cell adhesion molecule whose cytoplasmic domain contains a protein 4.1-binding motif (protein 4.1-BM) and a PDZ-binding motif (PDZ-BM). Loss of TSLC1 expression is frequently observed in advanced cancers implying its involvement in tumor invasion and/or metastasis. Using Madin-Darby canine kidney cells expressing a full-length TSLC1 or various cytoplasmic deletion mutants of TSLC1, we examined the role of TSLC1 in epithelial mesenchymal transitions during the hepatocyte growth factor (HGF)-induced tubulogenesis and cell scattering. In a three-dimensional culture, the full-length TSLC1, which was localized to the lateral membrane of Madin-Darby canine kidney cysts, inhibited HGF-induced tubulogenesis. In contrast, the mutants lacking either the protein 4.1-BM or the PDZ-BM abolished the inhibitory effect on tubulogenesis. In addition, these mutants showed aberrant subcellular localization indicating that lateral localization is correlated with the effect of TSLC1. In a two-dimensional culture, the full-length TSLC1, but not the mutants lacking the protein 4.1-BM or the PDZ-BM, suppressed HGF-induced cell scattering. Furthermore, the cells expressing full-length TSLC1 retained E-cadherin-based cell-cell adhesion even after being treated with HGF. These cells showed prolonged activation of Rac and low activity of Rho, whereas the HGF-treated parental cells induced transient activation of Rac and sustained activation of Rho. Prolonged Rac activation caused by the expression of TSLC1 required its cytoplasmic tail. These findings, taken together, suggest that TSLC1 plays a role in suppressing induction of epithelial mesenchymal transitions by regulating the activation of small Rho GTPases.

Rap1: a turnabout for the crosstalk between cadherins and integrins

The coordinate modulation of the cellular functions of cadherins and integrins plays an essential role in fundamental physiological and pathological processes, including morphogenesis, tissue differentiation and renewal, wound healing, immune surveillance, inflammatory response, tumour progression, and metastasis. However, the molecular mechanisms underlying the fine-balanced relationship between cadherin and integrin functions are still elusive. This review focuses on recent findings on the involvement of the small GTPase Rap1 in the regulation of cadherin- and integrin-dependent cell adhesion and signal transduction. In particular, it highlights some of the novel results recently obtained that raise the possibility of a pivotal role for Rap1 in the functional crosstalk between cadherins and integrins, suggesting interesting new regulatory mechanisms.

Endocytosis of the apical junctional complex: mechanisms and possible roles in regulation of epithelial barriers

Tight junctions (TJ) and adherens junctions (AJ) regulate cell-cell adhesion and barrier function of simple polarized epithelia. These junctions are positioned in the apical end of the lateral plasma membrane and form the so-called apical junctional complex (AJC). Although initially seen as purely structural features, the AJC is now known to play important roles in cell differentiation and proliferation. The AJC is a highly dynamic entity, undergoing rapid remodeling during normal epithelial morphogenesis and under pathologic conditions. There is growing evidence that remodeling of the AJC is mediated by internalization of junctional proteins. This review summarizes what is known about endocytic pathways, intracellular destinations and signaling cascades involved in internalization of AJC proteins. Potential biological roles for AJC endocytosis in maintaining functional apical junctions, reversible opening of epithelial barrier and disruption of intercellular adhesion are also discussed.

The Listeria protein internalin B mimics hepatocyte growth factor-induced receptor trafficking

Increased hepatocyte growth factor receptor (HGFR) signaling correlates closely with neoplastic invasion and metastatic potential of many human cancers. Hepatocyte growth factor receptor signaling is initiated by binding the physiological ligand HGF or the internalin B (InlB) protein of Listeria monocytogenes. Subsequent degradation of endocytosed HGFR terminates receptor signaling. Previously reported discrepancies in InlB and HGF-induced HGFR signaling could reflect differences in receptor internalization and degradation in response to these distinct ligands. We report that soluble InlB and HGF are mechanistically equivalent in triggering clathrin-dependent endocytosis and lysosomal degradation of HGFR. After internalization, InlB and HGF colocalize with Rab5, EEA1 and the transferrin receptor in classical early endosomes. Hepatocyte growth factor receptor internalization was prevented by overexpression of dominant negative mutants of dynamin 1 and epidermal growth factor phosphorylation substrate 15, but not caveolin 1, the GTPase Arf6 or the cholesterol-chelating drug Nystatin. Thus, HGFR internalization is principally clathrin-mediated and is not regulated by clathrin- independent pathways. Phosphatidylinositol 3-kinase signaling and HGF-regulated tyrosine kinase substrate were not required for ligand-triggered internalization of HGFR but were essential for subsequent lysosomal degradation. Thus, soluble InlB and HGF induce HGFR endocytosis and degradation by indistinguishable mechanisms, suggesting that InlB may be exploited to regulate pathogenic HGFR signaling.

The cytoplasmic expression of E-cadherin and beta-catenin in bovine trophoblasts during binucleate cell differentiation

Binucleate cells are endocrine cells generated by the acytokinesis and endoreduplication of the trophectoderm in the ruminant placenta. These cells are migratory and secrete hormones into the maternal circulation after fusing with uterine epithelial cells. In this study, we performed immunohistochemistry for E-cadherin and beta-catenin in bovine placenta and a bovine trophoblast cell line (BT-1). We found that E-cadherin and beta-catenin were distributed not only at the cell to cell boundary but throughout the cytoplasm in binucleate cells, although they were concentrated at the cell to cell boundary in epithelial cells in bovine placenta. Moreover, beta-catenin was detected in the nuclei of binucleate cells. Binucleate cells after fusion with uterine epithelial cells (feto-maternal hybrid cells) in the maternal side showed no intracellular expression of E-cadherin and beta-catenin. The transformation into binucleate cells in the BT-1 cell line was also accompanied by the cytoplasmic accumulation of E-cadherin and beta-catenin. We further demonstrated that levels of cytoplasmic beta-catenin were well correlated with the DNA content of binucleate cells in BT-1. The dynamic changes in the distribution of E-cadherin and beta-catenin suggest an important role in binucleate cells, including the rearrangement of cadherin-mediated cell adhesions during cell migration and the onset of endoreduplication probably via the nuclear transfer of beta-catenin.

Disassembling adherens junctions: breaking up is hard to do

Epithelial cells regulate their contacts with neighboring cells during embryonic development and in disease states such as tumor metastasis. The intercellular adherens junctions (AJs) are specialized subapical structures that function as principle mediators of cell-cell adhesion. Their disassembly correlates with a loss of cell-cell contact and an acquisition of migratory potential. Regulation of the expression of AJ components by gene transcription can dictate the stability of intercellular adhesive contacts, and accruing evidence indicates that the coordinated regulation of cellular processes such as membrane trafficking and cytoskeletal remodeling can also result in the effective dissolution of AJs. Studies of the posttranscriptional mechanisms by which adhesive interactions are regulated in response to growth factors and/or developmental cues have opened new avenues for investigating cell-cell adhesion during development and in disease.

Tumor cell pseudopodial protrusions. Localized signaling domains coordinating cytoskeleton remodeling, cell adhesion, glycolysis, RNA translocation, and protein translation

The pseudopodial protrusions of Moloney sarcoma virus (MSV)-Madin-Darby canine kidney (MDCK)-invasive (INV) variant cells were purified on 1-microm pore polycarbonate filters that selectively allow passage of the pseudopodial domains but not the cell body. The purified pseudopodial fraction contains phosphotyrosinated proteins, including Met and FAK, and various signaling proteins, including Raf1, MEK1, ERK2, PKBalpha (Akt1), GSK3alpha, GSK3beta, Rb, and Stat3. Pseudopodial proteins identified by liquid chromatography tandem mass spectrometry included actin and actin-regulatory proteins (ERM, calpain, filamin, myosin, Sra-1, and IQGAP1), tubulin, vimentin, adhesion proteins (vinculin, talin, and beta1 integrin), glycolytic enzymes, proteins associated with protein translation, RNA translocation, and ubiquitin-mediated protein degradation, as well as protein chaperones (HSP90 and HSC70) and signaling proteins (RhoGDI and ROCK). Inhibitors of MEK1 (U0126) and HSP90 (geldanamycin) significantly reduced MSV-MDCK-INV cell motility and pseudopod expression, and geldanamycin treatment inhibited Met phosphorylation and induced the expression of actin stress fibers. ROCK inhibition did not inhibit cell motility but transformed the pseudopodial protrusions of MSV-MDCK-INV cells into extended lamellipodia. Dominant negative Rho disrupted pseudopod expression and, in serum-starved cells, L-alpha-lysophosphatidic acid (oleoyl) activation of Rho induced pseudopodial protrusions or, in the presence of the ROCK inhibitor, extended lamellipodia. RNA was localized to the actin-rich pseudopodial domains of MSV-MDCK-INV cells, but the extent of colocalization with dense actin ruffles was reduced in the extended lamellipodia formed upon ROCK inhibition. Rho/ROCK activation in epithelial tumor cells therefore regulates RNA translocation to a pseudopodial domain that contains proteins involved in signaling, cytoskeleton remodeling, cell adhesion, glycolysis, and protein translation and degradation.

Semaphorin SEMA3F has a repulsing activity on breast cancer cells and inhibits E-cadherin-mediated cell adhesion

Previously, we demonstrated that loss of SEMA3F, a secreted semaphorin encoded in 3p21.3, is associated with higher stages in lung cancer and primary tumor cells studied with anti-vascular endothelial growth factor (VEGF) and SEMA3F antibodies. In vitro, SEMA3F inhibits cell spreading; this activity is opposed by VEGF. These results suggest that VEGF and SEMA3F compete for binding to their common neuropilin receptor. In the present report, we investigated the attractive/repulsive effects of SEMA3F on cell migration when cells were grown in a three-dimensional system and exposed to a SEMA3F gradient. In addition, we adapted the neurobiologic stripe assay to analyze the migration of tumor cells in response to SEMA3F. In the motile breast cancer cell line C100, which expresses both neuropilin-1 (NRP1) and neuropilin-2 (NRP2) receptors, SEMA3F had a repulsive effect, which was blocked by anti-NRP2 antibody. In less motile MCF7 cells, which express only NRP1, SEMA3F inhibited cell contacts with loss of membrane-associated E-cadherin and beta-catenin without motility induction. Cell spreading and proliferation were reduced. These results support the concept that in a first step during tumorigenesis, normal tissues expressing SEMA3F would try to prevent tumor cells from spreading and attaching to the stroma for further implantation.

Regulation of E-cadherin Endocytosis by Nectin through Afadin, Rap1, and p120ctn

Adherens junctions (AJs) are a major cell-cell adhesion structure in epithelial cells that are formed by two major cell-cell adhesion molecules, E-cadherin and nectin. We have previously shown that nectin first forms cell-cell adhesion and then recruits non-trans-interacting E-cadherin to the nectin-based cell-cell adhesion sites, which gradually trans-interact there, eventually forming AJs. We have examined here the effect of trans-interacting nectin on non-trans-interacting E-cadherin endocytosis. Trans-interacting nectin capable of associating with afadin, but not trans-interacting nectin mutant incapable of associating with afadin, inhibited non-trans-interacting E-cadherin endocytosis in intact cells. Afadin is a nectin- and actin filament-binding protein that connects nectin to the actin cytoskeleton. Studies on the mode of action of the nectin-afadin system using cell-free assay revealed that afadin associated with nectin bound Rap1 activated by trans-interacting nectin, interacted with p120ctn, and strengthened the binding of p120ctn to E-cadherin, eventually reducing non-trans-interacting E-cadherin endocytosis. Afadin, which did not bind Rap1, was inactive in this capacity. These results indicate that trans-interacting nectin inhibits non-trans-interacting E-cadherin endocytosis through afadin, Rap1, and p120ctn and thereby further accumulates non-trans-interacting E-cadherin to the nectin-based cell-cell adhesion sites for the formation of AJs.

Dynamics of cell adhesion and motility in living cells is altered by a single amino acid change in E-cadherin fused to enhanced green fluorescent protein

E-Cadherin regulates epithelial cell adhesion and is critical for the maintenance of tissue integrity. In sporadic diffuse-type gastric carcinoma, mutations of the E-cadherin gene are frequently observed that predominantly affect putative calcium binding motifs located in the linker region between the second and third extracellular domains. A single amino acid change (D370A) as found in a gastric carcinoma patient reduces cell adhesion and up-regulates cell motility. To study the effect of this mutation on the dynamics of cell adhesion and motility in living cells, enhanced green fluorescent protein (EGFP) was C-terminally fused to E-cadherin. The resulting mutant E-cadherin-EGFP fusion protein with a point mutation in exon 8 (p8-EcadEGFP) and a wild-type E-cadherin-EGFP fusion construct (wt-EcadEGFP) were expressed in human MDA-MB-435S cells. Fluorescent images were acquired by time-lapse laser scanning microscopy and E-cadherin was visualized during contact formation and in moving cells. Spatial and temporal localization of p8- and wt-EcadEGFP differed significantly. While wt-EcadEGFP was mainly localized at lateral membranes of contacting cells and formed E-cadherin puncta and plaques, p8-EcadEGFP-expressing cells frequently formed transient cell-cell contacts. During random cell migration, p8-EcadEGFP was found in lamellipodia. In contrast, wt-EcadEGFP localized at lateral cell-cell contact sites in low or non-motile cells. Inhibition of the epidermal growth factor (EGF) receptor, which plays a major role in lamellipodia formation and cell migration, reduced the motility of p8-EcadEGFP-expressing cells and caused lateral membrane staining of p8-EcadEGFP. Conversely, EGF induced cell motility and caused formation of lamellipodia that were E-cadherin positive. In conclusion, our data show that mutant E-cadherin significantly alters the dynamics of cell adhesion and motility in living cells and interferes with the formation of stable cell-cell contacts.

The ins and outs of E-cadherin trafficking

One way of controlling the activity of E-cadherin--a protein that is, simultaneously, a major cell-adhesion molecule, a powerful tumour suppressor, a determinant of cell polarity and a partner to the potent catenin signalling molecules--is to keep it on the move. During the past two decades, many insights into the fundamental role of E-cadherin in these processes have been garnered. Studies during the past five years have begun to reveal the importance of intracellular trafficking as a means of regulating the functions of E-cadherin. E-cadherin is trafficked to and from the cell surface by exocytic and multiple endocytic pathways. In this article, we survey the vesicle-trafficking machinery that is responsible for the sorting, transport, actin association and vesicle targeting of E-cadherin to regulate its movement and function during growth and development and, possibly, in cancer.

Lysosomal targeting of E-cadherin: a unique mechanism for the down-regulation of cell-cell adhesion during epithelial to mesenchymal transitions

A hallmark characteristic of epithelial tumor progression as well as some processes of normal development is the loss of the epithelial phenotype and acquisition of a motile or mesenchymal phenotype. Such epithelial to mesenchymal transitions are accompanied by the loss of E-cadherin function by either transcriptional or posttranscriptional mechanisms. Here we demonstrate that, upon v-Src expression, a potent trigger of epithelial to mesenchymal transitions, E-cadherin is internalized and then shuttled to the lysosome instead of being recycled back to the lateral membrane. Thus, while E-cadherin internalization facilitates the dissolution of adherens junctions, its subsequent traffic to the lysosome serves as a means to ensure that cells do not reform their cell-cell contacts and remain motile. We also show that ubiquitin tagging of E-cadherin is essential for its sorting to the lysosome. The lysosomal targeting of E-cadherin is mediated by hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) and v-Src-induced activation of the Rab5 and Rab7 GTPases. Our studies reveal that the lysosomal targeting of E-cadherin is an important posttranscriptional mechanism to deplete cellular E-cadherin during Src-induced epithelial to mesenchymal transitions.

Direct binding of Lgl2 to LGN during mitosis and its requirement for normal cell division

The Drosophila tumor suppressor protein lethal (2) giant larvae (l(2)gl) is involved in asymmetric cell division during development and epithelial cell polarity through interaction with the aPKC.Par-6 complex. We showed here that Lgl2, a mammalian homolog of l(2)gl, directly bound to LGN, a mammalian homolog of Partner of inscuteable in HEK293 cells. The C-terminal tail of Lgl2 bound to LGN with a K(d) value of about 56 nm. Endogenous Lgl2 formed a complex with aPKC, Par-6, and LGN. This complex formation was enhanced in metaphase of the synchronized cells by treatment with thymidine and nocodazole. Immunofluorescence staining of the complex was the strongest at the cell periphery of the metaphase cells. Overexpression of the C-terminal tail of Lgl2 induced mis-localization of the nuclear mitotic apparatus protein NuMA and disorganization of the mitotic spindle during mitosis, eventually causing formation of multiple micronuclei. Knockdown of endogenous Lgl (Lgl1 and Lgl2) also induced disorganization of the mitotic spindle, thereby causing formation of multiple micronuclei. The binding between Lgl2 and LGN played a role in the mitotic spindle organization through regulating formation of the LGN.NuMA complex. These results indicate that Lgl2 forms a Lgl2.Par-6.aPKC.LGN complex, which responds to mitotic signaling to establish normal cell division.