Posttranslational regulation of plakoglobin expression. Influence of the desmosomal cadherins on plakoglobin metabolic stability

Protein exchange is reduced in calcium-independent epithelial junctions

Desmosomes are cell–cell junctions that provide mechanical integrity to epithelial and cardiac tissues. Desmosomes have two distinct adhesive states, calcium-dependent and hyperadhesive, which balance tissue plasticity and strength. A highly ordered array of cadherins in the adhesive interface is hypothesized to drive hyperadhesion, but how desmosome structure confers adhesive state is still elusive. We employed fluorescence polarization microscopy to show that cadherin order is not required for hyperadhesion induced by pharmacologic and genetic approaches. FRAP experiments in cells treated with the PKCα inhibitor Gö6976 revealed that cadherins, plakoglobin, and desmoplakin have significantly reduced exchange in and out of hyperadhesive desmosomes. To test whether this was a result of enhanced keratin association, we used the desmoplakin mutant S2849G, which conferred reduced protein exchange. We propose that inside-out regulation of protein exchange modulates adhesive function, whereby proteins are “locked in” to hyperadhesive desmosomes while protein exchange confers plasticity on calcium-dependent desmosomes, thereby providing rapid control of adhesion.

Cleavage of Desmosomal Cadherins Promotes γ-Catenin Degradation and Benefits Wnt Signaling in Coxsackievirus B3-Induced Destruction of Cardiomyocytes

Coxsackievirus B3 (CVB3) is the primary etiologic agent of viral myocarditis, a major heart disease that occurs predominantly in children and young adolescents. In the heart, intercalated disks (ICD) are important structural formations that connect adjacent cardiomyocytes to maintain cardiac architecture and mediate signal communication. Deficiency in ICD components, such as desmosome proteins, leads to heart dysfunction. γ-catenin, a component protein of desmosomes, normally binds directly to desmocollin-2 and desmoglein-2. In this study, we found that CVB3 infection downregulated γ-catenin at the protein level but not the mRNA level in mouse HL-1 cardiomyocytes. We further found that this reduction of γ-catenin protein is a result of ubiquitin proteasome-mediated degradation, since the addition of proteasome inhibitor MG132 inhibited γ-catenin downregulation. In addition, we found that desmocollin-2 and desmoglein-2 were cleaved by both viral protease 3C and virus-activated cellular caspase, respectively. These cleavages led to the release of bound γ-catenin from the desmosome into the cytosol, resulting in rapid degradation of γ-catenin. Since γ-catenin shares high sequence homology with β-catenin in binding the TCF/LEF transcription factor, we further studied the effect of γ-catenin degradation on Wnt/β-catenin signaling. Luciferase assay showed that γ-catenin expression inhibited Wnt/β-catenin signaling. This finding was substantiated by qPCR to show that overexpression of γ-catenin downregulated transcription of Wnt signal target genes, c-myc and MMP9, while silencing γ-catenin upregulated these target genes. Finally, we demonstrated that γ-catenin expression inhibited CVB3 replication. In search for the underlying mechanism, we found that silencing γ-catenin caused down-regulation of interferon-β and its stimulated antiviral genes MDA5, MAVS, and ISG15. Taken together, our results indicate, for the first time, that CVB3 infection causes cardiomyocyte death through, at least in part, direct damage to the desmosome structure and reduction of γ-catenin protein, which in return promotes Wnt/β-catenin signaling and downregulates interferon-β stimulated immune responses.

The desmosome as a model for lipid raft driven membrane domain organization

Desmosomes are cadherin-based adhesion structures that mechanically couple the intermediate filament cytoskeleton of adjacent cells to confer mechanical stress resistance to tissues. We have recently described desmosomes as mesoscale lipid raft membrane domains that depend on raft dynamics for assembly, function, and disassembly. Lipid raft microdomains are regions of the plasma membrane enriched in sphingolipids and cholesterol. These domains participate in membrane domain heterogeneity, signaling and membrane trafficking. Cellular structures known to be dependent on raft dynamics include the post-synaptic density in neurons, the immunological synapse, and intercellular junctions, including desmosomes. In this review, we discuss the current state of the desmosome field and put forward new hypotheses for the role of lipid rafts in desmosome adhesion, signaling and epidermal homeostasis. Furthermore, we propose that differential lipid raft affinity of intercellular junction proteins is a central driving force in the organization of the epithelial apical junctional complex.

γ-Catenin at adherens junctions: mechanism and biologic implications in hepatocellular cancer after β-catenin knockdown

β-Catenin is important in liver homeostasis as a part of Wnt signaling and adherens junctions (AJs), while its aberrant activation is observed in hepatocellular carcinoma (HCC). We have reported hepatocyte-specific β-catenin knockout (KO) mice to lack adhesive defects as γ-catenin compensated at AJ. Because γ-catenin is a desmosomal protein, we asked if its increase in KO might deregulate desmosomes. No changes in desmosomal proteins or ultrastructure other than increased plakophilin-3 were observed. To further elucidate the role and regulation of γ-catenin, we contemplate an in vitro model and show γ-catenin increase in HCC cells upon β-catenin knockdown (KD). Here, γ-catenin is unable to rescue β-catenin/T cell factor (TCF) reporter activity; however, it sufficiently compensates at AJs as assessed by scratch wound assay, centrifugal assay for cell adhesion (CAFCA), and hanging drop assays. γ-Catenin increase is observed only after β-catenin protein decrease and not after blockade of its transactivation. γ-Catenin increase is associated with enhanced serine/threonine phosphorylation and abrogated by protein kinase A (PKA) inhibition. In fact, several PKA-binding sites were detected in γ-catenin by in silico analysis. Intriguingly γ-catenin KD led to increased β-catenin levels and transactivation. Thus, γ-catenin compensates for β-catenin loss at AJ without affecting desmosomes but is unable to fulfill functions in Wnt signaling. γ-Catenin stabilization after β-catenin loss is brought about by PKA. Catenin-sensing mechanism may depend on absolute β-catenin levels and not its activity. Anti-β-catenin therapies for HCC affecting total β-catenin may target aberrant Wnt signaling without negatively impacting intercellular adhesion, provided mechanisms leading to γ-catenin stabilization are spared.

γ-Catenin is overexpressed in acute myeloid leukemia and promotes the stabilization and nuclear localization of β-catenin

Canonical Wnt signaling regulates the transcription of T-cell factor (TCF)-responsive genes through the stabilization and nuclear translocation of the transcriptional co-activator, β-catenin. Overexpression of β-catenin features prominently in acute myeloid leukemia (AML) and has previously been associated with poor clinical outcome. Overexpression of γ-catenin mRNA (a close homologue of β-catenin) has also been reported in AML and has been linked to the pathogenesis of this disease, however, the relative roles of these catenins in leukemia remains unclear. Here we report that overexpression and aberrant nuclear localization of γ-catenin is frequent in AML. Significantly, γ-catenin expression was associated with β-catenin stabilization and nuclear localization. Consistent with this, we found that ectopic γ-catenin expression promoted the stabilization and nuclear translocation of β-catenin in leukemia cells. β-Catenin knockdown demonstrated that both γ- and β-catenin contribute to TCF-dependent transcription in leukemia cells. These data indicate that γ-catenin expression is a significant factor in the stabilization of β-catenin in AML. We also show that although normal cells exclude nuclear translocation of both γ- and β-catenin, this level of regulation is lost in the majority of AML patients and cell lines, which allow nuclear accumulation of these catenins and inappropriate TCF-dependent transcription.

Loss of desmocollin-2 confers a tumorigenic phenotype to colonic epithelial cells through activation of Akt/β-catenin signaling

This study provides evidence that decreased expression of the desmosomal cadherin desmocollin-2 enhances intestinal epithelial cell proliferation and promotes tumor formation via an Akt/β-catenin pathway.

Desmocollin-2 (Dsc2) and desmoglein-2 (Dsg2) are transmembrane cell adhesion proteins of desmosomes. Reduced expression of Dsc2 has been reported in colorectal carcinomas, suggesting that Dsc2 may play a role in the development and/or progression of colorectal cancer. However, no studies have examined the mechanistic contribution of Dsc2 deficiency to tumorigenesis. Here we report that loss of Dsc2 promotes cell proliferation and enables tumor growth in vivo through the activation of Akt/β-catenin signaling. Inhibition of Akt prevented the increase in β-catenin–dependent transcription and proliferation following Dsc2 knockdown and attenuated the in vivo growth of Dsc2-deficient cells. Taken together, our results provide evidence that loss of Dsc2 contributes to the growth of colorectal cancer cells and highlight a novel mechanism by which the desmosomal cadherins regulate β-catenin signaling.

Desmoglein-2: a novel regulator of apoptosis in the intestinal epithelium

Intestinal epithelial intercellular junctions regulate barrier properties, and they have been linked to epithelial differentiation and programmed cell death (apoptosis). However, mechanisms regulating these processes are poorly defined. Desmosomes are critical elements of intercellular junctions; they are punctate structures made up of transmembrane desmosomal cadherins termed desmoglein-2 (Dsg2) and desmocollin-2 (Dsc2) that affiliate with the underlying intermediate filaments via linker proteins to provide mechanical strength to epithelia. In the present study, we generated an antibody, AH12.2, that recognizes Dsg2. We show that Dsg2 but not another desmosomal cadherin, Dsc2, is cleaved by cysteine proteases during the onset of intestinal epithelial cell (IEC) apoptosis. Small interfering RNA-mediated down-regulation of Dsg2 protected epithelial cells from apoptosis. Moreover, we report that a C-terminal fragment of Dsg2 regulates apoptosis and Dsg2 protein levels. Our studies highlight a novel mechanism by which Dsg2 regulates IEC apoptosis driven by cysteine proteases during physiological differentiation and inflammation.

The prognostic value of E-cadherin and the cadherin-associated molecules alpha-, beta-, gamma-catenin and p120ctn in prostate cancer specific survival: a long-term follow-up study

OBJECTIVES

To determine the value of loss of expression of E-cadherin and cadherin associated molecules as prognostic markers for prostate cancer patients in a long-term follow-up study.

METHODS

Sixty-five prostate cancer specimens, obtained from patients with different stages of prostate cancer who underwent a radical prostatectomy or TUR-P between 1987 and 1991, were used for immunohistochemical analysis of the expression pattern of E-cadherin, alpha-, beta-, gamma-catenin and p120(ctn). Clinical records of these patients were studied for follow-up data and the prognostic value of expression of these adhesion molecules was determined by Kaplan-Meier survival analyses and multivariable proportional hazard regression analysis.

RESULTS

Normal staining patterns were found in 36 cases (55.4%) for E-cadherin, 37 cases (56.9%) for alpha-catenin, 40 cases (61.5%) for beta-catenin, 25 cases (38.5%) for gamma-catenin, and 40 cases (61.5%) for p120(ctn). Overall, a strong correlation was found between the expression of E-cadherin and other cadherin-associated molecules. The 5-year survival rates for each staining were as follows: E-cadherin (normal 79.2%, aberrant 26.8%), alpha-catenin (normal 79.2%, aberrant 26.8%), beta-catenin (normal 73.1%, aberrant 27.3%), gamma-catenin (normal 86.4%, aberrant 37.1%), and p120(ctn) (normal 72.8%, aberrant 30.0%). There was a significant difference in survival between normal and aberrant expression in each staining (log rank P < 0.0001). The proportional hazard regression model including tumor stage and Gleason score revealed alpha-catenin expression as the best prognostic marker for patients with prostate cancer.

CONCLUSIONS

Our data revealed a strong correlation between E-cadherin expression and other cadherin-associated molecules. Among these markers, alpha-catenin seems the best prognostic marker for prostate cancer specific survival. Larger studies are needed to confirm this result.

Increased keratinocyte proliferation initiated through downregulation of desmoplakin by RNA interference

The intercellular adhesive junction desmosomes are essential for the maintenance of tissue structure and integrity in skin. Desmoplakin (Dp) is a major obligate plaque protein which plays a fundamental role in anchoring intermediate filaments to desmosomal cadherins. Evidence from hereditary human disease caused by mutations in the gene encoding Dp, e.g. Dp haploinsufficiency, suggests that alterations in Dp expression result not only in the disruption of tissue structure and integrity but also could evoke changes in keratinocyte proliferation. We have used transient RNA interference (RNAi) to downregulate Dp specifically in HaCaT keratinocytes. We showed that this Dp downregulation also caused reduced expression of several other desmosomal proteins. Increased cell proliferation and enhanced G(1)-to-S-phase entry in the cell cycle, as monitored by colonial cellular density and BrdU incorporation, were seen in Dp RNAi-treated cells. These proliferative changes were associated with elevated phospho-ERK1/2 and phospho-Akt levels. Furthermore, this increase in phospho-ERK/1/2 and phospho-Akt levels was sustained in Dp RNAi-treated cells at confluence whereas in control cells there was a significant reduction in phosphorylation of ERK1/2. This study indicates that Dp may participate in the regulation of keratinocyte cell proliferation by, in part at least, regulating cell cycle progression.

The Differentiation-dependent Desmosomal Cadherin Desmoglein 1 Is a Novel Caspase-3 Target That Regulates Apoptosis in Keratinocytes

Although a number of cell adhesion proteins have been identified as caspase substrates, the potential role of differentiation-specific desmosomal cadherins during apoptosis has not been examined. Here, we demonstrate that UV-induced caspase cleavage of the human desmoglein 1 cytoplasmic tail results in distinct 17- and 140- kDa products, whereas metalloproteinase-dependent shedding of the extracellular adhesion domain generates a 75-kDa product. In vitro studies identify caspase-3 as the preferred enzyme that cleaves desmoglein 1 within its unique repeating unit domain at aspartic acid 888, part of a consensus sequence not conserved among the other desmosomal cadherins. Apoptotic processing leads to decreased cell surface expression of desmoglein 1 and re-localization of its C terminus diffusely throughout the cytoplasm over a time course comparable with the processing of other desmosomal proteins and cytoplasmic keratins. Importantly, whereas classic cadherins have been reported to promote cell survival, short hairpin RNA-mediated suppression of desmoglein 1 in differentiated keratinocytes protected cells from UV-induced apoptosis. Collectively, our results identify desmoglein 1 as a novel caspase and metalloproteinase substrate whose cleavage likely contributes to the dismantling of desmosomes during keratinocyte apoptosis and also reveal desmoglein 1 as a previously unrecognized regulator of apoptosis in keratinocytes.

HNF4alpha reduces proliferation of kidney cells and affects genes deregulated in renal cell carcinoma

Hepatocyte nuclear factor 4alpha (HNF4alpha) is a tissue-specific transcription factor known to regulate a large number of genes in hepatocytes and pancreatic beta cells. Although HNF4alpha is highly expressed in some sections of the kidney, little is known about its role in this organ and about HNF4alpha-regulated genes in the kidney cells. The abundance and activity of HNF4alpha are frequently reduced in renal cell carcinoma (RCC) indicating some tumor suppressing function of HNF4alpha in renal cells. To determine the potential role of HNF4alpha in RCC, we used Flp recombinase-mediated gene integration to generate human embryonic kidney cells (HEK293) that conditionally express wild-type or mutated HNF4alpha. Expression of wild-type HNF4alpha but not of the mutants led to reduction of proliferation and alterations of cell morphology. These effects were reversible and induced at physiological concentrations of HNF4alpha. Using gene expression profiling by microarrays, we determined genes regulated by HNF4alpha. Interestingly, many of the genes regulated by HNF4alpha have been shown to be deregulated in RCC microarray studies. These genes (ACY1, WT1, SELENBP1, COBL, EFHD1, AGXT2L1, ALDH5A1, THEM2, ABCB1, FLJ14146, CSPG2, TRIM9 and HEY1) are good candidates for genes whose activity is changed upon the decrease of HNF4alpha in RCC.

VE-cadherin increases the half-life of VEGF receptor 2

VE-cadherin plays a critical role in cell-cell interactions by forming adherens junctions in endothelial cells. VE-cadherin has increasingly been implicated in the cell signaling cascades initiated by the activation of growth factor receptors. Vascular endothelial growth factor receptor 2 (VEGFR-2) is present in regions of cell-cell contact and coimmunoprecipitates with VE-cadherin. In this study, we report that stable overexpression of VE-cadherin in two different endothelial cells induced an increase in VEGFR-2 protein levels. The increase in VEGFR-2 was also induced by overexpression of other classical cadherins such as E-cadherin or N-cadherin. Removing the extracellular domain of VE-cadherin abolished this effect, and a truncated form of VE-cadherin lacking the intracellular domain decreased VEGFR-2 instead of increasing it. VE-cadherin-induced changes in VEGFR-2 levels were paralleled by a corresponding shift in the VEGF-dependent activation of MAPK signaling, which demonstrated the functional relevance of varying the VEGFR-2 levels. Since VE-cadherin upregulated endogenous VEGFR-2 or exogenously expressed VEGFR-2, we hypothesized that the mechanism may be posttranslational. Indeed, the half-life of VEGFR-2 was 70 min in control cells whereas in cells overexpressing VE-cadherin the half-life was extended to 146 min. These results support the existence of a novel layer of functional regulation of VEGFR-2 by VE-cadherin.

Coordinated expression of desmoglein 1 and desmocollin 1 regulates intercellular adhesion

Desmoglein 1 (Dsg1) is a component of desmosomes present in the upper epidermis and can be targeted by autoimmune antibodies or bacterial toxins, resulting in skin blistering diseases. These defects in tissue integrity are believed to result from compromised desmosomal adhesion; yet, previous attempts to directly test the adhesive roles of desmosomal cadherins using normally non-adherent L cells have yielded mixed results. Here, two complementary approaches were used to better resolve the molecular determinants for Dsg1-mediated adhesion: (1) a tetracycline-inducible system was used to modulate the levels of Dsg1 expressed in L cell lines containing desmocollin 1 (Dsc1) and plakoglobin (PG) and (2) a retroviral gene delivery system was used to introduce Dsg1 into normal human epidermal keratinocytes (NHEK). By increasing Dsg1 expression relative to Dsc1 and PG, we were able to demonstrate that the ratio of Dsg1:Dsc1 is a critical determinant of desmosomal adhesion in fibroblasts. The distribution of Dsg1 was organized at areas of cell-cell contact in the multicellular aggregates that formed in these suspension cultures. Similarly, the introduction of Dsg1 into NHEKs was capable of increasing the aggregation of single cell suspensions and further enhanced the adhesive strength of intact epithelial sheets. Endogenous Dsc1 levels were also increased in NHEKs containing Dsg1, providing further support for the coordination of these two desmosomal cadherins in regulating adhesive structures. These Dsg1-mediated effects on intercellular adhesion were directly related to the presence of an intact extracellular domain as ETA, a toxin that specifically cleaves this desmosomal cadherin, inhibited adhesion in both fibroblasts and keratinocytes. Collectively, these observations demonstrate that Dsg1 promotes the formation of intercellular adhesion complexes and suggest that the relative level of Dsg and Dsc expressed at the cell surface regulates this adhesive process.

Cellular levels of p120 catenin function as a set point for cadherin expression levels in microvascular endothelial cells

The mechanisms by which catenins regulate cadherin function are not fully understood, and the precise function of p120 catenin (p120ctn) has remained particularly elusive. In microvascular endothelial cells, p120ctn colocalized extensively with cell surface VE-cadherin, but failed to colocalize with VE-cadherin that had entered intracellular degradative compartments. To test the possibility that p120ctn binding to VE-cadherin regulates VE-cadherin internalization, a series of approaches were undertaken to manipulate p120ctn availability to endogenous VE-cadherin. Expression of VE-cadherin mutants that competed for p120ctn binding triggered the degradation of endogenous VE-cadherin. Similarly, reducing levels of p120ctn using siRNA caused a dramatic and dose-related reduction in cellular levels of VE-cadherin. In contrast, overexpression of p120ctn increased VE-cadherin cell surface levels and inhibited entry of cell surface VE-cadherin into degradative compartments. These results demonstrate that cellular levels of p120ctn function as a set point mechanism that regulates cadherin expression levels, and that a major function of p120ctn is to control cadherin internalization and degradation.

Defining desmosomal plakophilin-3 interactions

Plakophilin 3 (PKP3) is a recently described armadillo protein of the desmosomal plaque, which is synthesized in simple and stratified epithelia. We investigated the localization pattern of endogenous and exogenous PKP3 and fragments thereof. The desmosomal binding properties of PKP3 were determined using yeast two-hybrid, coimmunoprecipitation and colocalization experiments. To this end, novel mouse anti-PKP3 mAbs were generated. We found that PKP3 binds all three desmogleins, desmocollin (Dsc) 3a and -3b, and possibly also Dsc1a and -2a. As such, this is the first protein interaction ever observed with a Dsc-b isoform. Moreover, we determined that PKP3 interacts with plakoglobin, desmoplakin (DP) and the epithelial keratin 18. Evidence was found for the presence of at least two DP-PKP3 interaction sites. This finding might explain how lateral DP-PKP interactions are established in the upper layers of stratified epithelia, increasing the size of the desmosome and the number of anchoring points available for keratins. Together, these results show that PKP3, whose epithelial and epidermal desmosomal expression pattern and protein interaction repertoire are broader than those of PKP1 and -2, is a unique multiprotein binding element in the basic architecture of a vast majority of epithelial desmosomes.

De novo formation of desmosomes in cultured cells upon transfection of genes encoding specific desmosomal components

Desmosomes are cell junctions and cytoskeleton-anchoring structures of epithelia, the myocardium, and dendritic reticulum cells of lymphatic follicles whose major components are known. Using cultured HT-1080 SL-1 fibrosarcoma-derived cells and transfection of cDNAs encoding specific desmosomal components, we have determined a minimum ensemble of proteins sufficient to introduce de novo structures, which, by morphology and functional competence, are indistinguishable from authentic desmosomes. In a more refined analysis, the influence of the desmosomal proteins desmoplakin (Dp), plakoglobin (Pg), and plakophilin 2 (Pp2) on the lateral clustering of the desmosomal transmembrane-glycoprotein desmoglein 2 (Dsg) was examined. We found that for efficient clustering of desmoglein 2 and desmosome structure formation, all three major plaque proteins-desmoplakin, plakoglobin, and plakophilin 2- were necessary. Furthermore, in this cell model, plakophilin 2 was capable of directing desmoplakin to adhaerens junctions (AJ), whereas plakoglobin was crucial for the segregation of desmosomal and AJ components. These results are discussed with respect to the variability in cell junction composition observed in various nonepithelial tissues.

Strict correlation between uPAR and plakoglobin expression in pemphigus vulgaris

BACKGROUND

Recent studies have reported nuclear delocalization of plakoglobin in acantholytic pemphigus vulgaris cells. The objective of this study was to evaluate the role of plakoglobin in the pathogenesis of acantholysis in pemphigus vulgaris (PV) and its relation with the urokinase-type plasminogen activator receptor (uPAR) expression.

MATERIALS AND METHODS

Plakoglobin and uPAR expressions were evaluated by immunohistochemistry in 22 cases of PV at various stages of the disease, and as controls in 18 specimens of skin/oral mucosa from healthy patients.

RESULTS

Healthy skin/normal oral mucosa showed strong plakoglobin expression in the basal and spinous layers with prevalent cellular membrane distribution; the intensity of staining progressively decreased toward the superficial layers of the epithelium. In PV patients, a progressive displacement of the plakoglobin signal toward the nucleus was found in 18/22 of the cases. Healthy skin/normal oral mucosa showed low uPAR expression with prevalent cellular membrane distribution. In the PV patients, strong uPAR expression was present in the acantholytic cells in 16/22 of the cases. There was direct correlation (p < 0.05) between the uPAR expression and nuclear plakoglobin.

CONCLUSIONS

The uPAR overexpression in acantholytic PV may be considered a direct consequence of plakoglobin abnormal distribution. Nuclear delocalization of plakoglobin, a direct consequence of plakoglobin-Dsg-3 dissociation induced by PV IgG, probably induces uPAR overexpression. This evidence suggests a central role for plakoglobin in PV pathogenesis because of its delocalization toward the nucleus, which is the probable cause of the uPAR gene expression.

Regulation of endothelial barrier function and growth by VE-cadherin, plakoglobin, and beta-catenin

VE-cadherin is an endothelial-specific cadherin that plays a central role in vascular barrier function and angiogenesis. The cytoplasmic domain of VE-cadherin is linked to the cytoskeleton through interactions with the armadillo family proteins beta-catenin and plakoglobin. Growing evidence indicates that beta-catenin and plakoglobin play important roles in epithelial growth and morphogenesis. To test the role of these proteins in vascular cells, a replication-deficient retroviral system was used to express intercellular junction proteins and mutants in the human dermal microvascular endothelial cell line (HMEC-1). A mutant VE-cadherin lacking an adhesive extracellular domain disrupted endothelial barrier function and inhibited endothelial growth. In contrast, expression of exogenous plakoglobin or metabolically stable mutants of beta-catenin stimulated HMEC-1 cell growth, which suggests that the beta-catenin signaling pathway was active in HMEC-1 cells. This possibility was supported by the finding that a dominant-negative mutant of the transcription factor TCF-4, designed to inhibit beta-catenin signaling, also inhibited HMEC-1 cell growth. These observations suggest that intercellular junction proteins function as components of an adhesion and signaling system that regulates vascular barrier function and growth.

Protein binding and functional characterization of plakophilin 2. Evidence for its diverse roles in desmosomes and beta -catenin signaling

Plakophilins are a subfamily of p120-related arm-repeat proteins that can be found in both desmosomes and the nucleus. Among the three known plakophilin members, plakophilin 1 has been linked to a genetic skin disorder and shown to play important roles in desmosome assembly and organization. However, little is known about the binding partners and functions of the most widely expressed member, plakophilin 2. To better understand the cellular functions of plakophilin 2, we have examined its protein interactions with other junctional molecules using co-immunoprecipitation and yeast two-hybrid assays. Here we show that plakophilin 2 can interact directly with several desmosomal components, including desmoplakin, plakoglobin, desmoglein 1 and 2, and desmocollin 1a and 2a. The head domain of plakophilin 2 is critical for most of these interactions and is sufficient to direct plakophilin 2 to cell borders. In addition, plakophilin 2 is less efficient than plakophilin 1 in localizing to the nucleus and enhancing the recruitment of excess desmoplakin to cell borders in transiently transfected COS cells. Furthermore, plakophilin 2 is able to associate with beta-catenin through its head domain, and the expression of plakophilin 2 in SW480 cells up-regulates the endogenous beta-catenin/T cell factor-signaling activity. This up-regulation by plakophilin 2 is abolished by ectopic expression of E-cadherin, suggesting that these proteins compete for the same pool of signaling active beta-catenin. Our results demonstrate that plakophilin 2 interacts with a broader repertoire of desmosomal components than plakophilin 1 and provide new insight into the possible roles of plakophilin 2 in regulating the signaling activity of beta-catenin.

Isoform-specific differences in the size of desmosomal cadherin/catenin complexes

Via their integration of the intermediate filament cytoskeleton into the cell membrane, desmosomes facilitate the maintenance of cell shape and tissue integrity as well as intercellular communication. The transmembrane components of the desmosome, the desmogleins and desmocollins, are members of the cadherin family of cell-cell adhesion molecules. Each of these proteins exists as three distinct isoforms, which are the products of individual genes and expressed in a cell-type and differentiation-specific manner. Previous work has suggested that desmoglein 1 binds to its catenin partner, plakoglobin, in an approximately 6:1 stoichiometry. In this study, the molecular organization of complexes formed by plakoglobin and desmoglein 1, 2, or 3 are further examined through immunoprecipitation, size exclusion chromatography and sucrose density sedimentation analysis. It is shown that the complex formed between plakoglobin and desmoglein 1 has an overall molecular weight greater than that of plakoglobin/desmoglein 2 or plakoglobin/desmoglein 3; however, the stoichiometry of the plakoglobin/desmoglein 1 complex does not appear to exceed 2:1.

Central role of the plakoglobin-binding domain for desmoglein 3 incorporation into desmosomes

The carboxy-termini of classical cadherins and desmocollins have been shown to play an important role in initiating desmosome assembly. In this study we wanted to determine whether the carboxy- terminal cytoplasmic domains of desmoglein 3 are important for targeting it to the desmosome. By generating stably transfected A431 cell lines with chimeric constructs encoding for the extracellular domain of E-cadherin and the transmembrane and intracellular region of human desmoglein 3, we could show that the cytoplasmic tail is sufficient to target the protein to the desmosome. By generating truncations of the carboxy-terminus we investigated the importance of the various intracellular subdomains. Whereas the construct encoding the intracellular cadherin-type segment domain still allowed its incorporation into the desmosome, further truncation, leaving only the intracellular anchor domain, did not. Deletion of the 87 amino acid long plakoglobin-binding site within the intracellular cadherin-type segment domain demonstrated that this region is essential for targeting desmoglein 3 to the desmosome. Absent the plakoglobin-binding site the chimeric molecule colocalizes with beta-catenin rather than desmoplakin. We conclude that binding of plakoglobin to desmoglein 3 is an important step in desmosome assembly and leads to the incorporation of desmoglein 3 into the desmosome.

A possible role of catenin dyslocalization in pemphigus vulgaris pathogenesis

BACKGROUND

Pemphigus vulgaris (PV) is an autoimmune blistering disease of the skin and mucosa due to the presence of autoantibodies against the components of desmosomes. To date, less is known about the expression levels of beta- and gamma-catenins in blistering diseases. The objective of this study was to evaluate the role of beta- and gamma-catenins in the pathogenesis of acantholysis in pemphigus vulgaris.

METHODS

beta- and gamma-catenin expression was evaluated by immunohistochemistry in 30 cases of PV at various stages of the disease and, as controls, in 18 specimens of the skin/oral mucosa of healthy patients.

RESULTS

Healthy skin and normal oral mucosa showed a strong beta- and gamma-catenin expression in basal and spinous layers with a prevalent cellular membrane distribution; the intensity of staining progressively decreased toward the superficial layers of epithelium. In PV patients, cytoplasmic expression of gamma-catenin was detected in 28/30 cases, and in 19/30 cases of PV for beta-catenin. Moreover, a progressive displacement of the signal toward the nucleus was found in 14/30 cases for beta-catenin, with dyslocalization toward the nucleus, particularly in areas with intense acantholysis, and in 22/30 cases of PV for gamma-catenin.

CONCLUSIONS

Abnormal distribution of gamma-catenin, consequent to PV IgG, may be considered a direct consequence of Dg3 dissociation from catenin. gamma-catenin likely plays a direct role in PV pathogenesis through its dyslocalization toward the nucleus or indirectly through the beta-catenin dyslocalization toward the nucleus, which is thought to induce transcription of selected target genes, such as uPAR.

Tyrosine-phosphorylated plakoglobin is associated with desmogleins but not desmoplakin after epidermal growth factor receptor activation

Tyrosine phosphorylation of junctional components has been proposed as a mechanism for modulating cell-cell adhesion. Although a correlation exists between the tyrosine phosphorylation of the adherens junction protein beta-catenin and loss of classical cadherin-mediated adhesion, the effects of tyrosine phosphorylation on the function of the adherens junction and desmosome-associated protein plakoglobin is unknown. In the present study, we investigated the effects of epidermal growth factor receptor (EGFR) tyrosine kinase activation on the subcellular distribution of plakoglobin and its association with its junctional binding partners. Long term epidermal growth factor (EGF) treatment of A431 cells revealed a modest decrease in the cytoskeleton-associated pool of plakoglobin (Pg) and a corresponding increase in the cytosolic pool of Pg. After short term EGF treatment, plakoglobin was rapidly phosphorylated, and tyrosine-phosphorylated Pg was distributed predominantly in a membrane-associated Triton X-100-soluble pool, along with a co-precipitating high molecular weight tyrosine-phosphorylated protein identified as desmoglein 2. Analysis of deletion and point mutants defined the primary EGFR-dependent targets as one or more of three C-terminal tyrosine residues. Whereas phosphorylated Pg remained associated with the desmoglein tail after both short and long term EGFR activation, no phosphorylated Pg was found associated with the N-terminal Pg-binding domain (DPNTP) of the intermediate filament-associated protein, desmoplakin. Together these results are consistent with the possibility that EGF-dependent tyrosine phosphorylation of Pg may modulate cell-cell adhesion by compromising the link between desmosomal cadherins and the intermediate filament cytoskeleton.

Assembly of desmosomal cadherins into desmosomes is isoform dependent

Desmosomes are intercellular adhesive junctions that exhibit cell- and differentiation-specific differences in their molecular composition. In complex epithelia, desmosomes contain multiple representatives of the desmosomal cadherin family, which includes three desmogleins and three desmocollins. Rules governing the assembly of desmosomal cadherin isoforms into desmosomes of different cell types are unknown. Here we compared the assembly properties of desmoglein 2 (Dsg2) and desmocollin 2 (Dsc2), which are widely expressed, with Dsg1 and Dsc1, which are expressed in the differentiated layers of complex epithelia, by introducing myc-tagged forms into simple and squamous epithelial cells that do not express Dsg1 or Dsc1. Dsc2.myc and Dsg2.myc assembled efficiently into desmosomes in every cell type in spite of significant shifts in the stoichiometric relationship between desmogleins and desmocollins. In contrast, Dsc1a.myc, Dsc1b.myc, and Dsg1.myc did not stably incorporate into desmosomes in any line. Coexpression of Dsc1a.myc or Dsc1b.myc and Dsg1.myc did not lead to their colocalization and failed to enhance incorporation of either cadherin into desmosomes. Dsg1.myc, but not Dsc1a, Dsc1b, disrupted desmosome assembly in a cell-type-specific manner, and disruption correlated with the recruitment of Dsg1.myc, but not Dsc1a or Dsc1b, into a Triton-insoluble pool. The plakoglobin:E-cadherin ratio decreased in Dsg1-expressing cells with disrupted desmosomes, but a decrease was also observed in a Dsc1a line. Thus, a modest reduction of plakoglobin associated with E-cadherin is apparently not sufficient to disrupt desmosome assembly. Our results demonstrate that desmosome assembly tolerates large shifts in cadherin stoichiometry, but is sensitive to isoform-specific differences exhibited by desmogleins and desmocollins.

Plakophilin 1 interferes with plakoglobin binding to desmoplakin, yet together with plakoglobin promotes clustering of desmosomal plaque complexes at cell-cell borders

Desmosomes are adhesive junctions that link intermediate filament networks to sites of strong intercellular adhesion. These junctions play an important role in providing strength to tissues that experience mechanical stress such as heart and epidermis. The basic structural elements of desmosomes are similar to those of the better-characterized adherens junctions, which anchor actin-containing microfilaments to cadherins at the plasma membrane. This linkage of actin to classic cadherins is thought to occur through an indirect mechanism requiring the associated proteins, alpha- and beta-catenin. In the case of desmosomes, both linear and lateral interactions have been proposed as playing an important role in formation of the plaque and linkage to the cytoskeleton. However, the precise nature of these interactions and how they cooperate in desmosome assembly are poorly understood. Here we employ a reconstitution system to examine the assembly of macromolecular complexes from components found in desmosomes of the differentiated layers of complex tissues. We demonstrate the existence of a Triton-soluble complex of proteins containing full length desmoplakin (DP), the arm protein plakoglobin, and the cytoplasmic domain of the desmosomal cadherin, desmoglein 1 (Dsg1). In addition, full length DP, but not an N-terminal plakoglobin binding domain of DP, co-immunoprecipitated with the Dsg1 tail in the absence of plakoglobin in HT1080 cells. The relative roles of the arm proteins plakoglobin and plakophilin 1 (PKP1) were also investigated. Our results suggest that, in the Triton soluble pool, PKP1 interferes with binding of plakoglobin to full length DP when these proteins are co-expressed. Nevertheless, both plakoglobin and PKP1 are required for the formation of clustered structures containing DP and the Dsg1 tail that ultrastructurally appear similar to desmosomal plaques found in the epidermis. These findings suggest that more than one armadillo family member is required for normal assembly and clustering of the desmosomal plaque in the upper layers of the epidermis.

Are desmosomes more than tethers for intermediate filaments?

Desmosomes are intercellular adhesive junctions that anchor intermediate filaments at membrane-associated plaques in adjoining cells, thereby forming a three-dimensional supracellular scaffolding that provides tissues with mechanical strength. But desmosomes have also recently been recognized as sensors that respond to environmental and cellular cues by modulating their assembly state and, possibly, their signalling functions.

Plakoglobin induces desmosome formation and epidermoid phenotype in N-cadherin-expressing squamous carcinoma cells deficient in plakoglobin and E-cadherin

Pg is a homologue of beta-catenin and Armadillo, the product of the Drosophila segment polarity gene and has been shown to have both adhesive and signaling functions. It interacts with both classic and desmosomal cadherins. Pg interaction with the desmosomal cadherins is essential for desmosome assembly. Its precise role in the classic cadherin complexes is unclear, although Pg-E-cadherin interaction appears to be necessary for the formation of desmosomes. In addition to cadherins in adhesion complexes, Pg interacts with a number of proteins involved in regulation of cell differentiation and proliferation such as Lef-1/Tcf-1 transcription factors and the tumor suppressor protein APC. In this study, we have introduced Pg cDNA into SCC9 cells, a Pg- and E-cadherin-deficient squamous cell carcinoma line, which also lacks desmosomes. These cells have both alpha-catenin and beta-catenin, display unusual expression of N-cadherin, and have the typical fibroblastic phenotype of transformed cells. Pg-expressing SCC9 cells (SCC9P) formed desmosomes. Desmosome formation coincided with the appearance of an epidermoid phenotype, with increased adhesiveness and a contact-dependent decrease in growth. Biochemical characterization of SCC9P cells showed an increase in the expression and stability of N-cadherin and a decrease in level and stability of beta-catenin, without any apparent effects on alpha-catenin. These results show that, in the absence of E-cadherin, Pg can efficiently use N-cadherin to induce desmosome formation and epidermoid phenotype. They also suggest a role for Pg as one of the regulators of the intracellular beta-catenin levels and underscore the pivotal role of this protein in regulating cell adhesion and differentiation.

Localizing the adhesive and signaling functions of plakoglobin

Plakoglobin (PKG) is a major component of cell-cell adhesive junctions. It is also closely related to the Drosophila segment polarity gene product armadillo and can induce a WNT-like neural axis duplication (NAD) phenotype in Xenopus [Kamovsky and Klymkowsky, 1995.] To define the regions of PKG involved in cell adhesion and inductive signaling, we examined the behavior of mutated forms of PKG in Xenopus. Deletion of amino acids 22 through 39 (in the Xenopus PKG sequence increased the apparent stability of the polypeptide within the embryo and increased its ability to induce a WNT-like, NAD phenotype when expressed in the vegetal hemisphere. The N-terminal "head" and first 6 "ARM" repeats of PKG, or the C-terminal "tail" and the last 3 "ARM" repeats, could be removed without destroying the remaining polypeptide's ability to induce a NAD phenotype. The nuclear localization of mutant PKGs, however, was not strictly correlated with the ability to induce a NAD phenotype, i.e., some inactive polypeptides still accumulate in nuclei. Removal of PKG's head and first ARM repeat, which includes its alpha-catenin binding site, resulted in a polypeptide that, when expressed in the embryo, generated alpha dramatic cell adhesion defect. Removal of the next three ARM repeats abolished this adhesion defect, suggesting that the polypeptide no longer competes effectively with endogenous catenins for binding to cadherins. Expression of a form of PKG truncated after the 5th ARM repeat produced a milder cell adhesion defect, whereas expression of a polypeptide truncated after the 8th ARM repeat had little apparent effect on cellular adhesion. Based on these observations, we conclude that functions related to stability and cellular adhesion reside in the N-terminal region of the polypeptide, whereas the ability to induce a NAD phenotype lies within repeats 6-10 of the central region. The function(s) of the C-terminal domain of PKG remain uncertain at this time.

Different effects of dominant negative mutants of desmocollin and desmoglein on the cell-cell adhesion of keratinocytes

Desmosomes contain two types of cadherin: desmocollin (Dsc) and desmoglein (Dsg). In this study, we examined the different roles that Dsc and Dsg play in the formation of desmosomes, by using dominant-negative mutants. We constructed recombinant adenoviruses (Ad) containing truncated mutants of E-cadherin, desmocollin 3a, and desmoglein 3 lacking a large part of their extracellular domains (EcaddeltaEC, Dsc3adeltaEC, Dsg3deltaEC), using the Cre-loxP Ad system to circumvent the problem of the toxicity of the mutants to virus-producing cells. When Dsc3adeltaEC Ad-infected HaCaT cells were cultured with high levels of calcium, E-cadherin and beta-catenin, which are marker molecules for the adherens junction, disappeared from the cell-cell contact sites, and cell-cell adhesion was disrupted. This also occurred in the cells infected with EcaddeltaEC Ad. With Dsg3deltaEC Ad infection, keratin insertion at the cell-cell contact sites was inhibited and desmoplakin, a marker of desmosomes, was stained in perinuclear dots while the adherens junctions remained intact. Dsc3adeltaEC Ad inhibited the induction of adherens junctions and the subsequent formation of desmosomes with the calcium shift, while Dsg3deltaEC Ad only inhibited the formation of desmosomes. To further determine whether Dsc3adeltaEC directly affected adherens junctions, mouse fibroblast L cells transfected with E-cadherin (LEC5) were infected with these mutant Ads. Both Dsc3adeltaEC and EcaddeltaEC inhibited the cell-cell adhesion of LEC5 cells, as determined by the cell aggregation assay, while Dsg3deltaEC did not. These results indicate that the dominant negative effects of Dsg3deltaEC were restricted to desmosomes, while those of Dsc3adeltaEC were observed in both desmosomes and adherens junctions. Furthermore, the cytoplasmic domain of Dsc3adeltaEC coprecipitated both plakoglobin and beta-catenin in HaCaT cells. In addition, beta-catenin was found to bind the endogenous Dsc in HaCaT cells. These findings lead us to speculate that Dsc interacts with components of the adherens junctions through beta-catenin, and plays a role in nucleating desmosomes after the adherens junctions have been established.

Differential interaction of plakoglobin and beta-catenin with the ubiquitin-proteasome system

Beta-catenin and plakoglobin are closely related armadillo family proteins with shared and distinct properties; Both are associated with cadherins in actin-containing adherens junctions. Plakoglobin is also found in desmosomes where it anchors intermediate filaments to the desmosomal plaques. Beta-catenin, on the other hand, is a component of the Wnt signaling pathway, which is involved in embryonic morphogenesis and tumorigenesis. A key step in the regulation of this pathway involves modulation of beta-catenin stability. A multiprotein complex, regulated by Wnt, directs the phosphorylation of beta-catenin and its degradation by the ubiquitin-proteasome system. Plakoglobin can also associate with members of this complex, but inhibition of proteasomal degradation has little effect on its levels while dramatically increasing the levels of beta-catenin. Beta-TrCP, an F-box protein of the SCF E3 ubiquitin ligase complex, was recently shown to play a role in the turnover of beta-catenin. To elucidate the basis for the apparent differences in the turnover of beta-catenin and plakoglobin we compared the handling of these two proteins by the ubiquitin-proteasome system. We show here that a deletion mutant of beta-TrCP, lacking the F-box, can stabilize the endogenous beta-catenin leading to its nuclear translocation and induction of beta-catenin/LEF-1-directed transcription, without affecting the levels of plakoglobin. However, when plakoglobin was overexpressed, it readily associated with beta-TrCP, efficiently competed with beta-catenin for binding to beta-TrCP and became polyubiquitinated. Fractionation studies revealed that about 85% of plakoglobin in 293 cells, is Triton X-100-insoluble compared to 50% of beta-catenin. These results suggest that while both plakoglobin and beta-catenin can comparably interact with beta-TrCP and the ubiquitination system, the sequestration of plakoglobin by the membrane-cytoskeleton system renders it inaccessible to the proteolytic machinery and stabilizes it.

Axin directly interacts with plakoglobin and regulates its stability

Plakoglobin is homologous to beta-catenin. Axin, a Wnt signal negative regulator, enhances glycogen synthase kinase (GSK)-3beta-dependent phosphorylation of beta-catenin and stimulates the degradation of beta-catenin. Therefore, we examined the effect of Axin on plakoglobin stability. Axin formed a complex with plakoglobin in COS cells and SW480 cells. Axin directly bound to plakoglobin, and this binding was inhibited by beta-catenin. Axin promoted GSK-3beta-dependent phosphorylation of plakoglobin. Furthermore, overexpression of Axin down-regulated the level of plakoglobin in SW480 cells. These results suggest that Axin regulates the stability of plakoglobin by enhancing its phosphorylation by GSK-3beta and that Axin may act on beta-catenin and plakoglobin in similar manners.

The head domain of plakophilin-1 binds to desmoplakin and enhances its recruitment to desmosomes. Implications for cutaneous disease

The contribution of desmosomes to epidermal integrity is evident in the inherited blistering disorder associated with the absence of a functional gene for plakophilin-1. To define the function of plakophilin-1 in desmosome assembly, interactions among the desmosomal cadherins, desmoplakin, and the armadillo family members plakoglobin and plakophilin-1 were examined. In transient expression assays, plakophilin-1 formed complexes with a desmoplakin amino-terminal domain and enhanced its recruitment to cell-cell borders; this recruitment was not dependent on the equimolar expression of desmosomal cadherins. In contrast to desmoplakin-plakoglobin interactions, the interaction between desmoplakin and plakophilin-1 was not mediated by the armadillo repeat domain of plakophilin-1 but by the non-armadillo head domain, as assessed by yeast two-hybrid and recruitment assays. We propose a model whereby plakoglobin serves as a linker between the cadherins and desmoplakin, whereas plakophilin-1 enhances lateral interactions between desmoplakin molecules. This model suggests that epidermal lesions in patients lacking plakophilin-1 are a consequence of the loss of integrity resulting from a decrease in binding sites for desmoplakin and intermediate filaments at desmosomes.

Immunocytochemical studies of the interactions of cadherins and catenins in the early Xenopus embryo

Linkage of cadherins to the cytoskeleton is crucial for their adhesive function. Since alpha- and beta-catenin play a key role in this linkage, these proteins are possible targets for processes that control cell-cell adhesion. To achieve a better understanding of the regulation of cell-cell adhesion in embryonic morphogenesis, we used immunohistology to investigate how in Xenopus blastomeres catenins respond to disturbances in the expression of maternal cadherins. Overexpression of myc-tagged maternal cadherin leads to a proportionate increase of the level of beta-catenin. The two proteins colocalize in the endoplasmic reticulum, in cytoplasmic vesicles, and along the cell membrane, indicating that the beta-catenin binds to overexpressed cadherin early in its passage to the plasma membrane. Expression of cadherin is essential for the stable presence of beta-catenin, as depletion from maternal cadherin mRNA leads to a complete loss of beta-catenin from the blastomeres. alpha-Catenin behaves differently. Overexpression of cadherin leaves the amount and localization of alpha-catenin largely unaffected, and additional cadherin inserts itself into the membrane without a proportionate rise in the level of membrane-bound alpha-catenin. However, cadherin mRNA depletion leads to a redistribution of alpha-catenin from the membrane to the cytoplasm. Thus, cadherin is required to localize alpha-catenin to the membrane, but the amount of alpha-catenin along the membrane seems to be restricted to a certain level which cannot be exceeded. The relevance of these observations for the regulation of cadherin-mediated cell adhesion in the Xenopus embryo is discussed. Additionally, we demonstrate that plakoglobin, like beta-catenin an armadillo repeat protein, shows neither accumulation after overexpression nor colocalization with the overexpressed cadherin.

Cadherin and catenin expression in normal human bronchial epithelium and non-small cell lung cancer

Cadherins are transmembrane cell adhesion molecules (CAMS) that mediate cell-cell interactions and are important for maintenance of epithelial cell integrity. This function is dependent on an indirect interaction between the cytoplasmic domain of the cadherin molecule with three cytoplasmic proteins known as alpha-, beta-, and gamma-catenin (-cat). Growing evidence suggests that alterations in cadherin or catenin expression or function may be important to the development of an invasive or metastatic phenotype. Immunohistochemical techniques were used to study the expression of the two major epithelial cadherins, E-cadherin (E-cad) and P-cadherin (P-cad) as well as alpha- and gamma-cat in normal bronchial epithelium and in a series of carefully TMN-staged pulmonary adenocarcinomas (n = 21) and squamous cell carcinomas (n = 7). The cadherin profile of normal pseudostratified bronchial epithelium was heterogeneous. Basilar cells strongly expressed P-cad, alpha- and gamma-cat, while columnar cells moderately expressed E-cad, alpha- and gamma-cat. In contrast to other epithelial tumors, E-cad on non-small cell lung carcinomas was actually upregulated, however, a decrease in P-cad expression was noted in 68%. At least one cadherin or catenin was downregulated, compared to normal bronchial epithelium, in 82% of tumors examined. With the exception of an association between loss of P-cad expression and poorly differentiated state, changes in cadherin and catenin expression levels were not significantly correlated to tumor stage, cell type, or nodal status. These findings illustrate that alteration of expression of cadherins and catenins are often found in non-small cell lung carcinoma when compared to the progenitor bronchial epithelium, and may play a role in the development of the malignant phenotype.

E-cadherin and alpha-, beta- and gamma-catenin expression in prostate cancers: correlation with tumour invasion

The E-cadherin-catenin complex plays an important role in establishing and maintaining intercellular connections and morphogenesis and reduced expression of its constituent molecules is associated with invasion and metastasis. In the present study, we examined E-cadherin and alpha-, beta- and gamma-catenin levels in tumour tissues obtained by radical prostatectomy in order to investigate the relationship with histopathological tumour invasion. Immunohistochemical findings for 45 prostate cancer specimens demonstrated aberrant expression of each molecule to be associated with dedifferentiation and, in addition, alteration of staining patterns for the three types of catenin was significantly correlated with capsular but not lymphatic or vascular invasion. The data thus suggest that three types of catenin may be useful predictive markers for biological aggressiveness of prostate cancer.

Desmosomal localization of beta-catenin in the skin of plakoglobin null-mutant mice

Plakoglobin, a protein belonging to the Armadillo-repeat gene family, is the only component that adherens junctions and desmosomes have in common. Plakoglobin null-mutant mouse embryos die because of severe heart defects and may exhibit an additional skin phenotype, depending on the genetic background. Lack of plakoglobin affects the number and structure of desmosomes, resulting in visible defects when cells are subjected to increasing mechanical stress, e.g. when embryonic blood starts circulating or during skin differentiation. By analysing plakoglobin-negative embryonic skin differentiation in more detail, we show here that, in the absence of plakoglobin, its closest homologue, beta-catenin, becomes localized to desmosomes and associated with desmoglein. This substitution may account for the relatively late appearance of the developmental defects seen in plakoglobin null-mutant embryos. beta-catenin cannot, however, fully compensate a lack of plakoglobin. In the absence of plakoglobin, there was reduced cell-cell adhesion, resulting in large intercellular spaces between keratinocytes, subcorneal acantholysis and necrosis in the granular layer of the skin. Electron microscopic analysis documented a reduced number of desmosomes, and those present lacked the inner dense plaque and had fewer keratin filaments anchored. Our analysis underlines the central role of plakoglobin for desmosomal assembly and function during embryogenesis.

Cadherin-mediated transmembrane interactions

We show in this study that cadherin ligands, either soluble or immobilized on different surfaces, can bind to cells carrying a compatible cadherin and induce long-range signals which affect cell adhesion and dynamics. Addition of recombinant N-cadherin extracellular domain (NEC) to CHO cells expressing N-cadherin (FL4) greatly enhanced the calcium-dependent aggregation of the cells and blocked their migration into an "in vitro wound". Monoclonal antibody which blocks cadherin interactions inhibited the aggregation of suspended FL4 cells and facilitated the "wound closure". As previously shown (Levenberg et al., 1998) synthetic beads coupled to NEC interacted specifically with the surface of FL4 cells and significantly enhanced the formation of adherens junctions. This effect was obtained also with the parental CHO cells, which contain low levels of N-cadherin, and in additional N-cadherin expressing cells such as cultured myoblasts. We further show here that stimulation of adhesion is not affected by the geometry of the NEC-bound surface and that cells plated on flat NEC-coated substratum also develop enhanced adherens junctions. Interaction of cells expressing low levels of endogenous N-cadherin, such as CHO cells with surface-immobilized N-cadherin ligands had a prominent effect also on the total level of N-cadherin and beta-catenin in the cells, probably due to stabilization of the cadherin-catenin complex by the interaction with the external surface.

VE-cadherin and desmoplakin are assembled into dermal microvascular endothelial intercellular junctions: a pivotal role for plakoglobin in the recruitment of desmoplakin to intercellular junctions

Vascular endothelial cells assemble adhesive intercellular junctions comprising a unique cadherin, VE-cadherin, which is coupled to the actin cytoskeleton through cytoplasmic interactions with plakoglobin, beta-catenin and alpha -catenin. However, the potential linkage between VE-cadherin and the vimentin intermediate filament cytoskeleton is not well characterized. Recent evidence indicates that lymphatic and vascular endothelial cells express desmoplakin, a cytoplasmic desmosomal protein that attaches intermediate filaments to the plasma membrane in epithelial cells. In the present study, desmoplakin was localized to intercellular junctions in human dermal microvascular endothelial cells. To determine if VE-cadherin could associate with desmoplakin, VE-cadherin, plakoglobin, and a desmoplakin amino-terminal polypeptide (DP-NTP) were co-expressed in L-cell fibroblasts. In the presence of VE-cadherin, both plakoglobin and DP-NTP were recruited to cell-cell borders. Interestingly, beta-catenin could not substitute for plakoglobin in the recruitment of DP-NTP to cell borders, and DP-NTP bound to plakoglobin but not beta-catenin in the yeast two-hybrid system. In addition, DP-NTP colocalized at cell-cell borders with alpha-catenin in the L-cell lines, and endogenous desmoplakin and alpha-catenin colocalized in cultured dermal microvascular endothelial cells. This is in striking contrast to epithelial cells, where desmoplakin and alpha -+catenin are restricted to desmosomes and adherens junctions, respectively. These results suggest that endothelial cells assemble unique junctional complexes that couple VE-cadherin to both the actin and intermediate filament cytoskeleton.

Desmosomes: intercellular adhesive junctions specialized for attachment of intermediate filaments

Cell-cell adhesion is thought to play important roles in development, in tissue morphogenesis, and in the regulation of cell migration and proliferation. Desmosomes are adhesive intercellular junctions that anchor the intermediate filament network to the plasma membrane. By functioning both as an adhesive complex and as a cell-surface attachment site for intermediate filaments, desmosomes integrate the intermediate filament cytoskeleton between cells and play an important role in maintaining tissue integrity. Recent observations indicate that tissue integrity is severely compromised in autoimmune and genetic diseases in which the function of desmosomal molecules is impaired. In addition, the structure and function of many of the desmosomal molecules have been determined, and a number of the molecular interactions between desmosomal proteins have now been elucidated. Finally, the molecular constituents of desmosomes and other adhesive complexes are now known to function not only in cell adhesion, but also in the transduction of intracellular signals that regulate cell behavior.

Molecular organization of the desmoglein-plakoglobin complex

Different epithelial intercellular junctions contain distinct complexes incorporating plakoglobin. In adherens junctions, plakoglobin interacts with two molecules, the transmembrane adhesion protein of the cadherin family (e.g. E-cadherin) and alpha-catenin. The latter is thought to anchor the cadherin-plakoglobin complex to the cortical actin cytoskeleton. In desmosomes, plakoglobin forms a complex with desmosomal cadherins, either desmoglein (Dsg) or desmocollin (Dsc), but not with alpha-catenin. To further understand the structure and assembly of the plakoglobin-cadherin complexes we analyzed amino acid residues involved in plakoglobin-Dsg interactions using alanine scanning mutagenesis. Previously, we have shown that plakoglobin interacts with a 72 amino acid-long cytoplasmic domain (C-domain) that is conserved among desmosomal and classic cadherins. In this paper, we show that a row of the large hydrophobic residues located at the C-terminal portion of the Dsg C-domain is indispensable for interaction with plakoglobin. To study a reciprocal site we expressed plakoglobin (MPg) or its mutants tagged by 6 myc epitope in epithelial A-431 cells. Using sucrose gradient centrifugation and subsequent co-immunoprecipitation, MPg was found to be efficiently incorporated into the same type of complexes as endogenous plakoglobin. A major pool of Dsg-plakoglobin complexes sedimented at 8S and exhibited a 1:1 stoichiometry. Using alanine scanning mutagenesis and the co-immunoprecipitation assay we identified nine hydrophobic amino acids within the arm repeats 1–3 of plakoglobin, that are required for binding to Dsg and Dsc. Eight of these amino acids also participate in the interaction with alpha-catenin. No mutations were found to reduce the affinity of plakoglobin binding to E-cadherin. These data provide direct evidence that the same hydrophobic plakoglobin surface is essential for mutually exclusive interaction with distinct proteins such as alpha-catenin and desmosomal cadherins.

The juxtamembrane region of the cadherin cytoplasmic tail supports lateral clustering, adhesive strengthening, and interaction with p120ctn

Cadherin cell-cell adhesion molecules form membrane-spanning molecular complexes that couple homophilic binding by the cadherin ectodomain to the actin cytoskeleton. A fundamental issue in cadherin biology is how this complex converts the weak intrinsic binding activity of the ectodomain into strong adhesion. Recently we demonstrated that cellular cadherins cluster in a ligand-dependent fashion when cells attached to substrata coated with the adhesive ectodomain of Xenopus C-cadherin (CEC1-5). Moreover, forced clustering of the ectodomain alone significantly strengthened adhesiveness (Yap, A.S., W.M. Brieher, M. Pruschy, and B.M. Gumbiner. Curr. Biol. 7:308-315). In this study we sought to identify the determinants of the cadherin cytoplasmic tail responsible for clustering activity. A deletion mutant of C-cadherin (CT669) that retained the juxtamembrane 94-amino acid region of the cytoplasmic tail, but not the beta-catenin-binding domain, clustered upon attachment to substrata coated with CEC1-5. Like wild-type C-cadherin, this clustering was ligand dependent. In contrast, mutant molecules lacking either the complete cytoplasmic tail or just the juxtamembrane region did not cluster. The juxtamembrane region was itself sufficient to induce clustering when fused to a heterologous membrane-anchored protein, albeit in a ligand-independent fashion. The CT669 cadherin mutant also displayed significant adhesive activity when tested in laminar flow detachment assays and aggregation assays. Purification of proteins binding to the juxtamembrane region revealed that the major associated protein is p120(ctn). These findings identify the juxtamembrane region of the cadherin cytoplasmic tail as a functionally active region supporting cadherin clustering and adhesive strength and raise the possibility that p120(ctn) is involved in clustering and cell adhesion.

Contributions of extracellular and intracellular domains of full length and chimeric cadherin molecules to junction assembly in epithelial cells

The integrity of cell-cell junctions in epithelial cells depends on functional interactions of both extracellular and intracellular domains of cadherins with other junction proteins. To examine the roles of the different domains of E-cadherin and desmoglein in epithelial junctions, we stably expressed full length desmoglein 1 and chimeras of E-cadherin and desmoglein 1 in A431 epithelial cells. Full length desmoglein 1 was able to incorporate into or disrupt endogenous desmosomes depending on expression level. Each of the chimeric cadherin molecules exhibited distinct localization patterns at the cell surface. A chimera of the desmoglein 1 extracellular domain and the E-cadherin intracellular domain was distributed diffusely at the cell surface while the reverse chimera, comprising the E-cadherin extracellular domain and the desmoglein 1 intracellular domain, localized in large, sometimes contiguous patches at cell-cell interfaces. Nevertheless, both constructs disrupted desmosome assembly. Expression of constructs containing the desmoglein 1 cytoplasmic domain resulted in approximately a 3-fold decrease in E-cadherin bound to plakoglobin and a 5- to 10-fold reduction in the steady-state levels of the endogenous desmosomal cadherins, desmoglein 2 and desmocollin 2, possibly contributing to the dominant negative effect of the desmoglein 1 tail. In addition, biochemical analysis of protein complexes in the stable lines revealed novel in vivo protein interactions. Complexes containing beta-catenin and desmoglein 1 were identified in cells expressing constructs containing the desmoglein 1 tail. Furthermore, interactions were identified between endogenous E-cadherin and the chimera containing the E-cadherin extracellular domain and the desmoglein 1 intracellular domain providing in vivo evidence for previously predicted lateral interactions of E-cadherin extracellular domains.

L-CAM expression induces fibroblast-epidermoid transition in squamous carcinoma cells and down-regulates the endogenous N-cadherin

SCC9 cells, derived from a squamous carcinoma of the tongue, were shown to lack E-cadherin but express alpha- and beta-catenins and N-cadherin. These cells also lack plakoglobin expression, do not assemble desmosomes and exhibit the typical morphology and growth properties of transformed cells. The N-cadherin expressed in SCC9 cells has properties similar to other classical cadherins, including interactions with the catenins. We transfected SCC9 cells with a full-length cDNA for L-CAM (liver cell adhesion molecule), the functional chicken homologue of E-cadherin. The exogenously expressed L-CAM formed complexes with catenins and the cytoskeleton and induced a morphological transition from fibroblastoid to epithelioid, conferred density-dependent growth inhibition, increased aggregation ability, and increased synthesis and stability of alpha- and beta-catenins. Coincident with these phenotypic changes, we detected a significant reduction in the level of endogenous N-cadherin, primarily as a result of rapid degradation of this protein in L-CAM-expressing cells. These results show the abnormal expression of N-cadherin in these transformed epidermoid cells, demonstrate the dynamics of the relationship between two cadherins, and provide a model system for the functional analysis of the tumor suppressor activity of E-cadherin in carcinomas.

Altered cell adhesion activity by pervanadate due to the dissociation of alpha-catenin from the E-cadherin.catenin complex

Leukemia cells (K562) that grow as non-adhesive single cells and have no endogenous cadherin were transfected with an E-cadherin expression vector, and cell clones stably expressing E-cadherin on their surface were established. The expression of E-cadherin induced the up-regulation of catenins, and E-cadherin became associated with catenins. The transfected cells grew as floating aggregates. Cell aggregation was Ca2+-dependent and was inhibited by E-cadherin antibodies. The aggregates dissociated into single cells on the addition of pervanadate. Pervanadate caused a dramatic augmentation of the phosphorylation of E-cadherin, beta-catenin, and gamma-catenin (plakoglobin), but alpha-catenin was not detectably phosphorylated. After pervanadate treatment, beta-catenin and gamma-catenin migrated more slowly on gel electrophoresis, suggesting changes in their conformations due to eventual changes in their phosphorylation levels. In the treated cells, a significant amount of alpha-catenin was dissociated from the E-cadherin.catenin complex. Aggregates of cells expressing an E-cadherin chimeric molecule covalently linked with alpha-catenin were not dissociated on pervanadate treatment, supporting the idea that the dissociation of alpha-catenin from the complex underlies the observed E-cadherin dysfunction.

Coexpression of both types of desmosomal cadherin and plakoglobin confers strong intercellular adhesion

Desmosomes are unique intercellular junctions in that they invariably contain two types of transmembrane cadherin molecule, desmocollins and desmogleins. In addition they possess a distinct cytoplasmic plaque structure containing a few major proteins including desmoplakins and the armadillo family member plakoglobin. Desmosomal cadherins are putative cell-cell adhesion molecules and we have tested their adhesive capacity using a transfection approach in mouse L cells. We find that L cells expressing either one or both of the desmosomal cadherins desmocollin 2a or desmoglein 1 display weak cell-cell adhesion activity that is Ca2+-dependent. Both homophilic and heterophilic adhesion could be detected. However, co-expression of plakoglobin with both desmosomal cadherins, but not with desmoglein 1 alone, resulted in a dramatic potentiation of cell-cell aggregation and the accumulation of detergent-insoluble desmosomal proteins at points of cell-cell contact. The effect of plakoglobin seems to be due directly to its interaction with the desmosomal cadherins rather than to its signalling function. The data suggest that the desmosome may obligatorily contain two cadherins and is consistent with a model in which desmocollins and desmogleins may form side by side heterodimers in contrast to the classical cadherins that are homodimeric. Plakoglobin may function by potentiating dimer formation, accretion of dimers to cell-cell contact sites or desmosomal cadherin stability.