Loss of density-dependent growth inhibition and dissociation of alpha-catenin from E-cadherin

Phosphatase regulation of intercellular junctions

Intercellular junctions represent the key contact points and sites of communication between neighboring cells. Assembly of these junctions is absolutely essential for the structural integrity of cell monolayers, tissues and organs. Disruption of junctions can have severe consequences such as diarrhea, edema and sepsis, and contribute to the development of chronic inflammatory diseases. Cell junctions are not static structures, but rather they represent highly dynamic micro-domains that respond to signals from the intracellular and extracellular environments to modify their composition and function. This review article will focus on the regulation of tight junctions and adherens junctions by phosphatase enzymes that play an essential role in preserving and modulating the properties of intercellular junction proteins.

WAVE2 Protein Complex Coupled to Membrane and Microtubules

E-cadherin is one of the key molecules in the formation of cell-cell adhesion and interacts intracellularly with a group of proteins collectively named catenins, through which the E-cadherin-catenin complex is anchored to actin-based cytoskeletal components. Although cell-cell adhesion is often disrupted in cancer cells by either genetic or epigenetic alterations in cell adhesion molecules, disruption of cell-cell adhesion alone seems to be insufficient for the induction of cancer cell migration and invasion. A small GTP-binding protein, Rac1, induces the specific cellular protrusions lamellipodia via WAVE2, a member of WASP/WAVE family of the actin cytoskeletal regulatory proteins. Biochemical and pharmacological investigations have revealed that WAVE2 interacts with many proteins that regulate microtubule growth, actin assembly, and membrane targeting of proteins, all of which are necessary for directional cell migration through lamellipodia formation. These findings might have important implications for the development of effective therapeutic agents against cancer cell migration and invasion.

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.

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.

Microsatellite analysis of the APC gene and immunoexpression of E-cadherin, catenin, and tubulin in esophageal squamous cell carcinoma

The histological features that accompany the development and progression of solid tumors are known to be controlled by a distinct cascade of molecular events. One such event is the inactivation of tumor suppressor genes, such as the adenomatous polyposis coli (APC) gene. Disruption of the cadherin-catenin cell adhesion complex also plays a role in the initial steps of cancer invasion and metastasis whereas alterations in cell structural molecules, such as tubulin, may contribute to the cancer phenotype. The understanding of the status of these molecules in ESSC may provide novel markers that could impact on management of the disease. The present study examined alterations in the microsatellite sequence of the APC gene via fluorescent-based polymerase chain reaction in 100 cases of primary esophageal squamous cell carcinoma. In addition, the expression of E-cadherin, alpha- and beta-catenin, and alpha- and beta-tubulin was analyzed using immunohistochemistry. These data were then statistically compared with each other as well as the relevant clinicopathologic data. Although the APC markers (D5S210, D5S346, D5S299, and D5S82) tested did show an overall high frequency of allelic imbalance/loss of heterozygosity (62.48%) and microsatellite instability (41.27%), they did not show prognostic significance in the study cohort and were not correlated with the immunohistochemical data. The tubulin proteins showed no significant change in expression in the tumor tissue The decreased immunoreactivity of E-cadherin was statistically correlated with the presence of lymph node metastases (P = .0180). Although alpha- and beta-catenin as well as E-cadherin showed no direct prognostic value, E-cadherin may warrant further investigation as an indirect prognostic indicator by allowing more accurate prediction of lymph node metastases.

Expression of cell adhesion molecules in oesophageal carcinoma and its prognostic value

Oesophageal carcinoma remains a disease of poor prognosis. Surgical cure rates are compromised by the fact that most patients are diagnosed at a late stage of disease because of the delayed onset of symptoms, by which time metastases and organ infiltration may have already occurred. Thus, invasion and metastases play a key role in influencing patient survival, and the search for novel treatments may therefore hinge on gaining insight into the mechanisms controlling these processes. It has been established that the initial step in the metastatic cascade is the detachment of tumour cells from the primary tumour via dysregulation of normal cell-cell and cell-matrix interactions. Distinct proteins known as cell adhesion molecules (CAMs) mediate these interactions. In recent years, a plethora of information has contributed to the in depth understanding of these molecules. This review provides a brief description of five families of CAMs (cadherins, integrins, CD44, immunoglobulin superfamily, and selectins) and highlights their altered expression in relation both to prognosis and tumour behaviour in squamous cell carcinoma and adenocarcinoma of the oesophagus.

Reduced cell adhesion during mitosis by threonine phosphorylation of beta1 integrin

Cell shape and adhesion of cultured mammalian cells change dramatically during mitosis, however, how cell cycle-dependent alterations in cell adhesion are regulated remain to be elucidated. We show here that normal human mammary epithelial (HME) cells which became less adhesive and adopted the rounded morphology during the G(2)/M phase of the cell cycle significantly reduced their dependence on beta1 integrin-mediated adhesion to laminin, by using function blocking antibody to beta1 integrin. In G(2)/M cells, both total and cell surface expressions of beta1 integrin were comparable with those in G(1) cells but it was phosphorylated at threonines 788-789 within its cytoplasmic domain and coimmunoprecipitated Ca(2+)/calmodulin-dependent protein kinase (CaMK) II. The threonine phosphorylated beta1 integrin significantly reduced its intracellular linkage with actin, with no significant reduction in the actin expression. In contrast, beta1 integrin in G(1) cells was not threonine phosphorylated but formed a link with actin and coimmunoprecipitated the core enzyme of the serine/threonine protein phosphatase (PP) 2A. The results suggest that reduced beta1 integrin-mediated cell adhesion of HME cells to the substratum during mitosis may be induced by beta1 integrin phosphorylation at threonines 788-789 and its reduced ability to link with the actin cytoskeleton.

Endothelial barrier strengthening by activation of focal adhesion kinase

Endothelial cell barrier (EC) properties regulate blood tissue fluid flux. To determine the role of endothelial-matrix interactions in barrier regulation, we induced cell shrinkage by exposing confluent endothelial monolayers to hyperosmolarity. The dominant effect of a 15-min hyperosmolar exposure was an increase in the trans-endothelial electrical resistance, indicating the induction of barrier strengthening. Hyperosmolar exposure also increased activity of focal adhesion kinase and E-cadherin accumulation at the cell periphery. Concomitantly, the density of actin filaments increased markedly. In EC monolayers stably expressing constitutively active or dominant negative isoforms of Rac1, the actin response to hyperosmolar exposure was enhanced or blocked, respectively, although the response in trans-endothelial resistance was unaffected, indicating that the endothelial barrier enhancement occurred independently of actin. However, in monolayers expressing a kinase-deficient mutant of focal adhesion kinase, the hyperosmolarity-induced increases in activity of focal adhesion and peripheral E-cadherin enhancement were blocked and the induced increase of electrical resistance was markedly blunted. These findings indicate that in EC exposed to hyperosmolar challenge, the involvement of focal adhesion kinase was critical in establishing barrier strengthening.

Functional role of alpha-actinin, PI 3-kinase and MEK1/2 in insulin-like growth factor I receptor kinase regulated motility of human breast carcinoma cells

Within epithelial tissue, cells are held together by specialized lateral junctions. At particular stages of development and in pathological processes such as metastasis, cells break down the intercellular junctions, separate from the epithelial sheet and migrate individually. Despite the importance of these processes, little is understood about the regulatory mechanisms of active cell separation. In view of the effects of insulin-like growth factor I (IGF-I) on mammary gland development and cancer, we developed a model using MCF-7 human breast cancer cells in which the process of cell separation can be induced by IGF-I. The separation was enhanced in MCF-7 cells overexpressing the IGF-IR and blocked in the cells expressing a dead-kinase mutant of this receptor. Activation of the IGF-IR resulted in a rapid formation of motile actin microspikes at the regions of cell-cell contacts, disorganization of mature adherens junctions and the onset of cell migration. In cell separation, the signaling between the IGF-IR kinase and actin required phosphatidylinositol 3 (PI 3)-kinase-generated phospholipids but not MAP kinases and was mediated by alpha-actinin. The activity of MEK1/2 kinases was needed for consecutive cell migration. This work also defined a new function for alpha-actinin. Upon IGF-IR activation, green fluorescence protein (GFP)-labeled alpha-actinin concentrated at the base of actin microspikes. Deletion of the N-terminal actin-binding domain of alpha-actinin prevented this redistribution, indicating that this domain is necessary. Detection of the C-terminal tail of alpha-actinin reduced the number of microspikes, showing that alpha-actinin has a role in the development of microspikes and is not passively reorganized with filamentous actin. We suggest that the signaling pathway from the IGF-IR kinase through the PI-3 kinase to alpha-actinin participates in the rapid organization of actin into microspikes at the cell-cell junctions and leads to active cell separation, whereas signaling through ERK1/2 MAP kinases controls cell migration following cell separation.

Interaction between EGFR signaling and DE-cadherin during nervous system morphogenesis

Dynamically regulated cell adhesion plays an important role during animal morphogenesis. Here we use the formation of the visual system in Drosophila embryos as a model system to investigate the function of the Drosophila classic cadherin, DE-cadherin, which is encoded by the shotgun (shg) gene. The visual system is derived from the optic placode which normally invaginates from the surface ectoderm of the embryo and gives rise to two separate structures, the larval eye (Bolwig’s organ) and the optic lobe. The optic placode dissociates and undergoes apoptotic cell death in the absence of DE-cadherin, whereas overexpression of DE-cadherin results in the failure of optic placode cells to invaginate and of Bolwig’s organ precursors to separate from the placode. These findings indicate that dynamically regulated levels of DE-cadherin are essential for normal optic placode development. It was shown previously that overexpression of DE-cadherin can disrupt Wingless signaling through titration of Armadillo out of the cytoplasm to the membrane. However, the observed defects are likely the consequence of altered DE-cadherin mediated adhesion rather than a result of compromising Wingless signaling, as overexpression of a DE-cadherin-α-catenin fusion protein, which lacks Armadillo binding sites, causes similar defects as DE-cadherin overexpression. We further studied the genetic interaction between DE-cadherin and the Drosophila EGF receptor homolog, EGFR. If EGFR function is eliminated, optic placode defects resemble those following DE-cadherin overexpression, which suggests that loss of EGFR results in an increased adhesion of optic placode cells. An interaction between EGFR and DE-cadherin is further supported by the finding that expression of a constitutively active EGFR enhances the phenotype of a weak shg mutation, whereas a mutation in rhomboid (rho) (an activator of the EGFR ligand Spitz) partially suppresses the shg mutant phenotype. Finally, EGFR can be co-immunoprecipitated with anti-DE-cadherin and anti-Armadillo antibodies from embryonic protein extracts. We propose that EGFR signaling plays a role in morphogenesis by modulating cell adhesion.

Regulation of VE-cadherin linkage to the cytoskeleton in endothelial cells exposed to fluid shear stress

Endothelial cells exposed to shear stress realigned and elongated in the direction of flow through the coordinated remodeling of their adherens junctions and actin cytoskeleton. The elaborate networks of VE-cadherin complexes in static cultures became more uniform and compact in response to shear. In contrast, the cortical actin present in static cultures was reorganized into numerous stress fiber bundles distributed parallel to the direction of flow. Exposure to shear did not significantly alter the expression of the junctional proteins VE-cadherin, beta-catenin, and alpha-catenin, but the composition of the junctional complexes did change. We detected a marked decrease in the alpha-catenin associated with VE-cadherin complexes in endothelial monolayers subjected to shear. This loss of alpha-catenin, the protein that links beta-catenin-bound cadherin to the actin cytoskeleton, was not due to decreased quantities of beta-catenin associated with VE-cadherin. Instead, the loss of alpha-catenin from the junctional complexes coincided with the increased tyrosine phosphorylation of beta-catenin associated with VE-cadherin. The change in beta-catenin phosphorylation closely correlated with the shear-induced loss of the protein tyrosine phosphatase SHP-2 from VE-cadherin complexes. Thus, the functional interaction of alpha-catenin with VE-cadherin-bound beta-catenin is regulated by the extent of tyrosine phosphorylation of beta-catenin. This, concomitantly, is regulated by SHP-2 associated with VE-cadherin complexes.

The linkage between beta1 integrin and the actin cytoskeleton is differentially regulated by tyrosine and serine/threonine phosphorylation of beta1 integrin in normal and cancerous human breast cells

BACKGROUND

Structural requirements for the beta1 integrin functions in cell adhesion, spreading and signaling have been well documented mainly for fibroblasts. In this study, we examined the reason for the reduced surface expression of beta1 integrin in human breast cancer MCF-7 cells compared to normal human breast epithelial (HBE) cells, both of which adhered to collagen type IV.

RESULTS

The beta1 integrin immunoprecipitates from either HBE or MCF-7 cells involved alpha-actinin while actin coprecipitated with beta1 integrin from HBE cells but not from MCF-7 cells. Immunoblotting using the anti-phosphotyrosine (PY) antibody indicated the phosphorylation of beta1 integrin at least at tyrosine in both cells. Dephosphorylation of beta1 integrin from HBE cells by protein tyrosine phosphatase (PTP), but not by protein serine/threonine phosphatase (PP), caused dissociation of actin from beta1 integrin, although dephosphorylation of it from MCF-7 cells by either PTP or PP caused association of the two proteins. In MCF-7 cells beta1 integrin coprecipitated doublet of proteins having the Ca2+/calmodulin-dependent protein kinase (CaMK) II activity that was susceptible to KN-62, a specific inhibitor of CaMKII.

CONCLUSION

The results suggest that beta1 integrin is tyrosine phosphorylated and links with actin via alpha-actinin in HBE cells but prevented from linking with actin in MCF-7 cells by phosphorylation at both tyrosine and serine/threonine of beta1 integrin which forms a complex with alpha-actinin and CaMKII. Thus the linkage formation of beta1 integrin with actin may be differentially regulated by its tyrosine and serine/threonine phosphorylation in normal HBE cells and breast cancer MCF-7 cells.

Actin filament assembly and actin-myosin contractility are necessary for anchorage- and EGF-dependent activation of phospholipase Cgamma

Formation of actin stress fibers and the focal adhesion complex between cell and the substratum are crucial for nonmalignant cells to achieve anchorage-dependent growth. We show here that the adhesion complex formed in normal human mammary epithelial (HME) cells which adhered to type IV collagen, involved the EGF receptor (EGFR) and phospholipase Cgamma (PLCgamma) as signaling molecules, in addition to integrin beta1, alpha-actinin, and actin even before stimulation of the cells with EGF. Stimulation of cells with EGF induced tyrosine phosphorylation of EGFR and activation of PLCgamma, as assessed by the production of a second messenger diacylglycerol (DAG), without any significant increase in the amount of EGFR-bound PLCgamma. Disruption of either actin filaments by cytochalasin D (CD) or actin-myosin contractility by ML-7, an inhibitor of myosin light chain kinase (MLCK), altered the flattened morphology of quiescent cells to a retracted one, without affecting the association between EGFR and PLCgamma. Stimulation of CD- or ML-7-treated cells with EGF failed to inhibit tyrosine phosphorylation of EGFR and its association and colocalization with PLCgamma, but inhibited the PLCgamma activation. Phosphatidylinositol 4,5-bisphosphate (PtdInsP2), substrate of PLCgamma, was tightly associated with alpha-actinin and the content of alpha-actinin-bound PtdInsP2 was reduced by treatment of cells with ML-7 but not with CD. The amount of PtdInsP2 bound to alpha-actinin was increased by the addition of EGF and this EGF-induced increase was blocked by either CD or ML-7. The present results suggest that anchorage-dependent EGF signaling in HME cells may require both actin filament assembly and actin-myosin contractility for the PLCgamma activation.

Involvement of IQGAP1, an effector of Rac1 and Cdc42 GTPases, in cell-cell dissociation during cell scattering

We have previously proposed that IQGAP1, an effector of Rac1 and Cdc42, negatively regulates cadherin-mediated cell-cell adhesion by interacting with beta-catenin and by causing the dissociation of alpha-catenin from cadherin-beta-catenin-alpha-catenin complexes and that activated Rac1 and Cdc42 positively regulate cadherin-mediated cell-cell adhesion by inhibiting the interaction of IQGAP1 with beta-catenin. However, it remains to be clarified in which physiological processes the Rac1-Cdc42-IQGAP1 system is involved. We here examined whether the Rac1-IQGAP1 system is involved in the cell-cell dissociation of Madin-Darby canine kidney II cells during 12-O-tetradecanoylphorbol-13-acetate (TPA)- or hepatocyte growth factor (HGF)-induced cell scattering. By using enhanced green fluorescent protein (EGFP)-tagged alpha-catenin, we found that EGFP-alpha-catenin decreased prior to cell-cell dissociation during cell scattering. We also found that the Rac1-GTP level decreased after stimulation with TPA and that the Rac1-IQGAP1 complexes decreased, while the IQGAP1-beta-catenin complexes increased during action of TPA. Constitutively active Rac1 and IQGAP1 carboxyl terminus, a putative dominant-negative mutant of IQGAP1, inhibited the disappearance of alpha-catenin from sites of cell-cell contact induced by TPA. Taken together, these results indicate that alpha-catenin is delocalized from cell-cell contact sites prior to cell-cell dissociation induced by TPA or HGF and suggest that the Rac1-IQGAP1 system is involved in cell-cell dissociation through alpha-catenin relocalization.

E-cadherin-catenin cell-cell adhesion complex and human cancer

BACKGROUND

The E-cadherin-catenin complex plays a crucial role in epithelial cell-cell adhesion and in the maintenance of tissue architecture. Perturbation in the expression or function of this complex results in loss of intercellular adhesion, with possible consequent cell transformation and tumour progression. Recently, much progress has been made in understanding the interaction between the different components of this protein complex and how this cell-cell adhesion complex is modulated in cancer cells.

METHODS

This is an update of the role of the E-cadherin-catenin complex in human cancers. It emphasizes new features and the possible role of the complex in clinical practice, discussed in the light of 165 references obtained from the Medline database from 1995 to 1999.

RESULTS

More evidence is now appearing to suggest that disturbance in protein-protein interaction in the E-cadherin-catenin adhesion complex is one of the main events in the early and late steps of cancer development. An inverse correlation is found between expression of the E-cadherin-catenin complex and the invasive behaviour of tumour cells. Therefore, E-cadherin-catenin may become a significant prognostic marker for tumour behaviour. Besides its role in establishing tight cell-cell adhesion, beta- catenin plays a major role in cell signalling and promotion of neoplastic growth. This suggests its dual role as a tumour suppressor and as an oncogene in human cancers.

CONCLUSION

Recent developments show that the E-cadherin-catenin complex is more than a 'sticky molecular complex'. Further studies may yield greater insight into the early molecular interactions critical to the initiation and progression of tumours. This should aid the development of novel strategies for both prevention and treatment of cancer.

Anchorage-independent activation of mitogen-activated protein kinase through phosphatidylinositol-3 kinase by insulin-like growth factor I

Insulin-like growth factor I (IGF-I) is a well-established mitogen in human breast cancer cells. We show here that human breast cancer MCF-7 cells, which were prevented from attaching to the substratum and were floating in medium, responded to IGF-I and initiated DNA synthesis. The addition of IGF-I to floating cells induced activation of protein kinase B (PKB)/Akt, as to cells attached to the substratum. In addition, mitogen-activated protein kinase (MAPK)/extracellular response kinase (ERK) and its upstream kinases, ERK kinase (MEK) and Raf-1, were activated by IGF-I in floating cells. While the IGF-I-induced activation of PKB/Akt was inhibited by PI3-K inhibitor LY294002 but not by MEK inhibitor PD98059, the activation of both MEK and ERK by IGF-I was inhibited by both. These findings suggest that the IGF-I signal that leads to stimulation of DNA synthesis of MCF-7 cells is transduced to ERK through PI3-K, only when they are anchorage-deficient.

Expression of Ep-CAM in normal, regenerating, metaplastic, and neoplastic liver

Ep-CAM is a homophilic, Ca2+-independent cell-cell adhesion molecule that is expressed in many human epithelial tissues. Its increased expression is closely associated with active cell proliferation. Furthermore, in epithelial cell types that in adults lack Ep-CAM (i. e. squamous epithelia), up-regulation of Ep-CAM coincides with the early stages of neoplastic change. This study has analysed the expression of Ep-CAM in liver, in the hepatocytes and cells of the biliary duct system, in relation to proliferative diseases and carcinogenesis. Adult hepatocytes are Ep-CAM negative, with only bile duct epithelium being positive in the liver tissue. However, in the 8-week embryonic liver, the majority of hepatocytes express Ep-CAM. During regeneration and repair of liver tissues associated with focal nodular hyperplasia and (biliary) cirrhosis, activation of Ep-CAM expression was observed, with high expression levels in so-called 'ductular proliferations'-regenerating stem cells. During precursor cell differentiation into mature hepatocytes, several intermediate morphological stages could be observed, all Ep-CAM positive, including cells morphologically close to mature hepatocytes. Full maturation of the precursor resulted in the disappearance of Ep-CAM expression. The results suggest that expression of Ep-CAM is a prerequisite of the proliferative phenotype during differentiation of hepatocyte precursors. In liver neoplasia, Ep-CAM was expressed in almost all cholangiocarcinomas (10/11), whereas the majority of hepatocellular carcinomas (8/10) were negative, suggesting that malignant proliferation of hepatocellular carcinoma cells is not related to expression of Ep-CAM and that hepatocellular carcinoma originates from a highly differentiated precursor. The results indicate that Ep-CAM can be used as an additional immunohistochemical marker to distinguish cholangiocarcinoma from hepatocellular carcinoma due to the differential expression of Ep-CAM in these tumours.

SHP2 association with VE-cadherin complexes in human endothelial cells is regulated by thrombin

Thrombin-mediated changes in endothelial cell adherens junctions modulate vascular permeability. We demonstrate that the nonreceptor protein-tyrosine phosphatase SHP2 co-precipitates with VE-cadherin complexes in confluent, quiescent human umbilical vein endothelial cells. Ligand-binding blots using a SHP2-glutathione S-transferase fusion peptide established that SHP2 associates selectively with beta-catenin in VE-cadherin complexes. Thrombin treatment of human umbilical vein endothelial cells promotes SHP2 tyrosine phosphorylation and dissociation from VE-cadherin complexes. The loss of SHP2 from the cadherin complexes correlates with a dramatic increase in the tyrosine phosphorylation of beta-catenin, gamma-catenin, and p120-catenin complexed with VE-cadherin. We propose that thrombin regulates the tyrosine phosphorylation of VE-cadherin-associated beta-catenin, gamma-catenin, and p120-catenin by modulating the quantity of SHP2 associated with VE-cadherin complexes. Such changes in adherens junction complex composition likely underlie thrombin-elicited alterations in endothelial monolayer permeability.

Rac regulates the stability of the adherens junction and its components, thus affecting epithelial cell differentiation and transformation

We have previously reported that activated rac (V12rac) can bring about hypertransformation of ras-transformed epithelial cells, which can be suppressed by the dominant negative form of rac (N17rac). Starting with primary epithelial cells, a series of cell lines expressing wild type (WT) or mutated forms of ras or rac were generated and analysed for their adhesive function and expression and association of adherens junction (AJ) proteins. Normal, primary epithelial cells were self-adhesive and expressed AJs that were very stable. The expression of constitutively active ras resulted in a decrease in, but not loss of, cell-cell adhesion, with concomitantly decreased stability of AJ components. This was extremely exacerbated by the co-expression of constitutively activate rac, but was suppressed by dominant negative rac, which resulted in increased cell-cell adhesion and extremely stable AJs. alpha-catenin also failed to associate with E-cadherin-beta-catenin complexes in cells expressing V12rac. Expression of V12rac resulted in the loss of epithelial morphology. The extent of transformation of each cell type corresponded to the stability of the respective AJ complexes. Thus, rac seems to be involved in regulating the stability of AJs, which promote epithelial cell differentiation, and consequently, modulating tumor progression.

Cdc42, Rac1, and their effector IQGAP1 as molecular switches for cadherin-mediated cell-cell adhesion

Cell-cell adhesion is a dynamic process in various cellular and developmental situations. Cadherins, well-known Ca(2+)-dependent adhesion molecules, are thought to play a major role in the regulation of cell-cell adhesion. However, the molecular mechanism underlying the rearrangement of cadherin-mediated cell-cell adhesion is largely unknown. Cdc42 and Rac1, belonging to the Rho small GTPase family, have recently been shown to be involved in the regulation of cell-cell adhesion. In addition, IQGAP1, an effector for Cdc42 and Rac1, has been shown to regulate the cadherin function through interaction with beta-catenin, a molecule associated with cadherin. In this review, we will summarize the mode of action of Cdc42 and Rac1 as well as IQGAP1 as molecular switches for the cadherin function, and then discuss physiological processes in which the Cdc42/Rac1/IQGAP1 system may be involved.

Expression of Ep-CAM in normal, regenerating, metaplastic, and neoplastic liver

Ep-CAM is a homophilic, Ca2+-independent cell-cell adhesion molecule that is expressed in many human epithelial tissues. Its increased expression is closely associated with active cell proliferation. Furthermore, in epithelial cell types that in adults lack Ep-CAM (i. e. squamous epithelia), up-regulation of Ep-CAM coincides with the early stages of neoplastic change. This study has analysed the expression of Ep-CAM in liver, in the hepatocytes and cells of the biliary duct system, in relation to proliferative diseases and carcinogenesis. Adult hepatocytes are Ep-CAM negative, with only bile duct epithelium being positive in the liver tissue. However, in the 8-week embryonic liver, the majority of hepatocytes express Ep-CAM. During regeneration and repair of liver tissues associated with focal nodular hyperplasia and (biliary) cirrhosis, activation of Ep-CAM expression was observed, with high expression levels in so-called 'ductular proliferations'-regenerating stem cells. During precursor cell differentiation into mature hepatocytes, several intermediate morphological stages could be observed, all Ep-CAM positive, including cells morphologically close to mature hepatocytes. Full maturation of the precursor resulted in the disappearance of Ep-CAM expression. The results suggest that expression of Ep-CAM is a prerequisite of the proliferative phenotype during differentiation of hepatocyte precursors. In liver neoplasia, Ep-CAM was expressed in almost all cholangiocarcinomas (10/11), whereas the majority of hepatocellular carcinomas (8/10) were negative, suggesting that malignant proliferation of hepatocellular carcinoma cells is not related to expression of Ep-CAM and that hepatocellular carcinoma originates from a highly differentiated precursor. The results indicate that Ep-CAM can be used as an additional immunohistochemical marker to distinguish cholangiocarcinoma from hepatocellular carcinoma due to the differential expression of Ep-CAM in these tumours.

alpha-Catenin-vinculin interaction functions to organize the apical junctional complex in epithelial cells

alphaE-catenin, a cadherin-associated protein, is required for tight junction (TJ) organization, but its role is poorly understood. We transfected an alphaE-catenin-deficient colon carcinoma line with a series of alphaE-catenin mutant constructs. The results showed that the amino acid 326-509 domain of this catenin was required to organize TJs, and its COOH-terminal domain was not essential for this process. The 326-509 internal domain was found to bind vinculin. When an NH2-terminal alphaE-catenin fragment, which is by itself unable to organize the TJ, was fused with the vinculin tail, this chimeric molecule could induce TJ assembly in the alphaE-catenin-deficient cells. In vinculin-null F9 cells, their apical junctional organization was impaired, and this phenotype was rescued by reexpression of vinculin. These results indicate that the alphaE-catenin-vinculin interaction plays a role in the assembly of the apical junctional complex in epithelia.

Role of IQGAP1, a target of the small GTPases Cdc42 and Rac1, in regulation of E-cadherin- mediated cell-cell adhesion

The small guanosine triphosphatases (GTPases) Cdc42 and Rac1 regulate E-cadherin-mediated cell-cell adhesion. IQGAP1, a target of Cdc42 and Rac1, was localized with E-cadherin and beta-catenin at sites of cell-cell contact in mouse L fibroblasts expressing E-cadherin (EL cells), and interacted with E-cadherin and beta-catenin both in vivo and in vitro. IQGAP1 induced the dissociation of alpha-catenin from a cadherin-catenin complex in vitro and in vivo. Overexpression of IQGAP1 in EL cells, but not in L cells expressing an E-cadherin-alpha-catenin chimeric protein, resulted in a decrease in E-cadherin-mediated cell-cell adhesive activity. Thus, IQGAP1, acting downstream of Cdc42 and Rac1, appears to regulate cell-cell adhesion through the cadherin-catenin pathway.