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Cadherin genes encode a superfamily of conserved transmembrane proteins that share an adhesive ectodomain composed of tandem cadherin repeats. More than 100 human cadherin superfamily members have been identified, which can be classified into three families: major cadherins, protocadherins and cadherin-related proteins. These superfamily members are involved in diverse fundamental cellular processes including cell-cell adhesion, morphogenesis, cell recognition and signaling. Epithelial cadherin (E-cadherin) is the founding cadherin family member. Its cytoplasmic tail interacts with the armadillo catenins, p120 and β-catenin. Further, α-catenin links the cadherin/armadillo catenin complex to the actin filament network. Even genomes of ancestral metazoan species such as cnidarians and placozoans encode a limited number of distinct cadherins and catenins, emphasizing the conservation and functional importance of these gene families. Moreover, a large expansion of the cadherin and catenin families coincides with the emergence of vertebrates and reflects a major functional diversification in higher metazoans. Here, we revisit and review the functions, phylogenetic classifications and co-evolution of the cadherin and catenin protein families., (Copyright © 2017 Elsevier Inc. All rights reserved.)

The alphaE-catenin is a well-known invasion suppressor. A recently described novel alpha-catenin, i.e. alphaT-catenin (CTNNA3), shows related functions being necessary for the formation of cell-cell adhesion complexes. We recently demonstrated that the 10q21.3 region containing the CTNNA3 gene shows a parent-of-origin effect and that transcription of the CTNNA3 gene is downregulated in placental tissues of complete androgenetic origin. As this suggests that the CTNNA3 gene is subject to imprinting, we performed allele-specific RT-PCR on early placenta tissues using informative heterozygous samples. This was supplemented by immunostaining for alphaT-catenin, p57KIP2 and low molecular weight cytokeratin in tissues of a partial hydatidiform mole. As shown here we demonstrate that the CTNNA3 gene is subject to imprinting with preferential expression of the maternal allele in first trimester placental tissues. Imprinting, however, is trophoblast cell type-dependent: expression in extravillus trophoblast is biallelic; expression in villus cytotrophoblast is from the maternal allele only. Expression of alphaT-catenin is lost in villus syncytiotrophoblast as well as in extravillus trophoblast following epithelial-mesenchymal transition. The trophoblast cell type-dependent imprinting of CTNNA3 is identical to p57KIP2 imprinting with respect to trophoblast cell type (villus) and parental origin of the expressed allele (maternal). This suggests that gene dosage compensation of CTNNA3 and p57KIP2 in the placenta shares a conserved regulatory mechanism that correlates with an early step in trophoblast determination, i.e. differentiation into villus or extravillus trophoblast.

AlphaT-catenin is a recently identified member of the alpha-catenin family of cell-cell adhesion molecules. Its expression is restricted mainly to cardiomyocytes, although it is also expressed in skeletal muscle, testis and brain. Like other alpha-catenins, alphaT-catenin provides an indispensable link between a cadherin-based adhesion complex and the actin cytoskeleton, resulting in strong cell-cell adhesion. We show here that the tissue-specificity of alphaT-catenin expression is controlled by its promoter region. By in silico analysis, we found that the alphaT-catenin promoter contains several binding sites for cardiac and muscle-specific transcription factors. By co-transfection studies in P19 embryonal carcinoma cells, we demonstrated that MEF2C and GATA-4 each have an activating effect on the alphaT-catenin promoter. Transfections with wild-type and mutant promoter constructs in cardiac HL-1 cells indicated that one GATA box is absolutely required for high alphaT-catenin promoter activity in these cells. Furthermore, we showed that the GATA-4 transcription factor specifically binds and activates the alphaT-catenin promoter in vivo in cardiac HL-1 cells. In vivo promoter analysis in transgenic mice revealed that the isolated alphaT-catenin promoter region could direct the tissue-specific expression of a LacZ reporter gene in concordance with endogenous alphaT-catenin expression.

Morphological plasticity of dendritic spines and synapses is thought to be crucial for their physiological functions. Here we show that alpha N-catenin, a linker between cadherin adhesion receptors and the actin cytoskeleton, is essential for stabilizing dendritic spines in rodent hippocampal neurons in culture. In the absence of alpha N-catenin, spine heads were abnormally motile, actively protruding filopodia from their synaptic contact sites. Conversely, alpha N-catenin overexpression in dendrites reduced spine turnover, causing an increase in spine and synapse density. Tetrodotoxin (TTX), a neural activity blocker, suppressed the synaptic accumulation of alpha N-catenin, whereas bicuculline, a GABA antagonist, promoted it. Furthermore, excess alpha N-catenin rendered spines resistant to the TTX treatment. These results suggest that alpha N-catenin is a key regulator for the stability of synaptic contacts.

The mechanisms by which growth factors cooperate with cell adhesion molecules to modulate epithelial cell motility remain poorly understood. Here, we investigated the role of the E-cadherin/catenin complex in insulin-like growth factor (IGF-I)-dependent cell migration and invasion. We used variants of the HCT-8 colon cancer family that differ in their expression of alphaE-catenin, an intracellular molecule that links the E-cadherin/catenin complex to the actin cytoskeleton. Migration was determined using a monolayer wound model and cell invasion by the penetration of the cells into type-I collagen gels. We showed that alpha-catenin-deficient cells were not able to migrate in cohort upon IGF-I stimulation. Transfection of these cells with alpha-catenin isoforms (alphaN- or alphaT-catenin) restored migratory response IGF-I. These results suggest that alpha-catenins are involved in the signal issued from the E-cadherin/catenin complex to regulate IGF-I-stimulated migration. In contrast, IGF-I promoted invasion of both alpha-catenin-deficient and alpha-catenin-expressing cells, indicating that alpha-catenin did not participate in the regulation of IGF-I-induced invasion. Inhibition of E-cadherin function by treatment with MB-2 monoclonal antibodies inhibited both IGF-I-dependent cell migration and invasion. Taken together, our results indicate that functional alpha-catenin is essential for migration but not for invasion, while E-cadherin is involved in both phenomena.

The gene encoding alpha-T-catenin, CTNNA3, is positioned within a region on chromosome 10, showing strong evidence of linkage to Alzheimer's disease (AD), and is therefore a good positional candidate gene for this disorder. We have demonstrated that alpha-T-catenin is expressed in human brain, and like other alpha-catenins, it inhibits Wnt signaling and is therefore also a functional candidate. We initially genotyped two single-nucleotide polymorphisms (SNPs) in the gene, in four independent samples comprising over 1200 cases and controls but failed to detect an association with either SNP. Similarly, we found no evidence for association between CTNNA3 and AD in a sample of subjects showing linkage to chromosome 10, nor were these SNPs associated with Abeta deposition in brain. To comprehensively screen the gene, we genotyped 30 additional SNPs in a subset of the cases and controls (n > 700). None of these SNPs was associated with disease. Although an excellent candidate, we conclude that CTNNA3 is unlikely to account for the AD susceptibility locus on chromosome 10.

Alpha T-catenin is a novel member of the alpha-catenin family, which shows most abundant expression in cardiomyocytes and in peritubular myoid cells of the testis, pointing to a specific function for alpha T-catenin in particular muscle tissues. Like other alpha-catenins, alpha T-catenin provides an indispensable link between the cadherin-based cell-cell adhesion complex and the cytoskeleton, to mediate cell-cell adhesion. By isolating genomic clones, combined with database sequence analysis, we have been able to determine the structure of the CTNNA3 and Ctnna3 genes, encoding human and mouse alpha T-catenin, respectively. The positions of the exon-exon boundaries are completely conserved in CTNNA3, Ctnna3, and the alpha N-catenin encoding CTNNA2 gene. They overlap largely with the boundaries of the CTNNA1 and CTNNAL1 genes encoding alpha E-catenin and alpha-catulin, respectively. This emphasizes that these alpha-catenin genes evolved from the same ancestor gene. Nevertheless, the introns of CTNNA3 and Ctnna3 are remarkably large (often more than 100 kb) compared with introns of other CTNNA genes. The CTNNA3 gene was mapped to chromosome band 10q21 by both fluorescence in situ hybridization and polymerase-chain-reaction-based hybrid mapping. This region encodes a gene for autosomal dominant familial dilated cardiomyopathy (DCM), a common cause of morbidity and mortality. As alpha T-catenin is highly expressed in healthy heart tissue, we have considered CTNNA3 as a candidate disease gene in a family showing DCM linkage to the 10q21-q23 locus. Mutation screening of all 18 exons of the CTNNA3 gene in this family has, however, not detected any DCM-linked CTNNA3 mutations.

Abnormal expression of the alphaE-catenin protein, a component of the E-cadherin/catenin cell adhesion complex, is frequently observed in human cancer cells. An inverse correlation between alphaE-catenin expression and tumor malignancy can be of prognostic value. Mutations of the alphaE-catenin gene, CTNNA1, were described in several human cancer cell lines and were found to result in aberrant cell adhesion. We have developed a polymerase chain reaction/single-strand conformation polymorphism-based method for mutation analysis of this gene in human tumor DNA. This approach enabled us to identify several polymorphisms in a set of desmoid tumors, demonstrating that this method is suitable for alphaE-catenin mutational analysis. On the basis of our genomic characterization data, we found that the previously reported alternative splicing of the alphaE-catenin gene actually generates a frame-shift, resulting in a truncated alphaE-catenin protein. This finding is unlike the other alpha-catenin family members alphaN-catenin and vinculin, which show in-frame alternative inserts. Furthermore, real-time quantitative reverse transcriptase-PCR analysis did not reveal relevant expression levels of this alternatively spliced alphaE-catenin variant neither in any human tissue or cell line tested, nor at any mouse developmental stage tested. Thus, contrary to previous notions, alternative splicing with in-frame insertion nearby the C-terminal end of the protein is not a general feature for all members of the alpha-catenin/vinculin family.

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.

Cadherins are major cell-cell adhesion proteins whose cytoplasmic domains bind to catenin proteins. Strong intercellular adhesion depends on linkage of the cadherin/catenin complex to the actin cytoskeleton via alpha-catenin. To date, it is not clear how different cell types achieve the variable strength of cell-cell adhesion clearly needed in a multicellular organism. Here, we report the cloning and molecular characterization of alphaT(testis)-catenin, a novel human cDNA encoding a protein with homology to both human alphaE(epithelial)-catenin and alphaN(neural)-catenin. Although originally discovered in testis, alphaT-catenin is expressed in other tissues, the highest levels being observed in heart. Immunohistochemical analysis showed human alphaT-catenin localization at intercalated discs of cardiomyocytes and in peritubular myoid cells of testis. In cells transfected with alphaT-catenin cDNA, interaction with beta-catenin was demonstrated by co-immunoprecipitation. Transfection of alpha-catenin-deficient colon carcinoma cells recruited E-cadherin and beta-catenin to cell-cell contacts and functional cadherin-mediated cell-cell adhesion was restored in this way. Moreover, compaction of these cells was at least as prominent as in the case of cells expressing endogenous alphaE-catenin. We propose that alphaT-catenin is necessary for the formation of stretch-resistant cell-cell adhesion complexes, in particular, muscle cells.

Neuroendocrine tumours (NETs) of the lung represent a wide spectrum of phenotypically distinct entities, with differences in tumour progression and aggressiveness. The redistribution and/or the loss of various cell adhesion molecules, such as the E-cadherin-catenin complex, play a predominant role in carcinogenesis and in tumour invasion. Moreover, mutations in exon 3 of the beta-catenin gene, the adenomatous polyposis coli (APC) gene or the E-cadherin genes were previously found to result in intracytoplasmic and/or nuclear beta-catenin protein accumulation, activating nuclear transcription of target genes involved in tumour progression. In the present study, the distribution of the components of this E-cadherin-catenin complex has been investigated by immunohistochemistry and an attempt has been made to correlate the abnormal expression pattern with the eventual detection of mutations in the corresponding genes. This study included 27 primary NETs of the lung, with nine typical carcinoids (TCs), three atypical carcinoids (ACs), and 15 large cell neuroendocrine carcinomas (LCNECs). The E-cadherin-catenin complex remained expressed in most of these lung tumours, but with a cytoplasmic and/or nuclear redistribution of beta-catenin, E-cadherin, and alpha-catenin; abnormal positive immunoreactivity was observed in 24 (88.9%), in 21 (80.8%), and in 20 (76.9%) NETs, respectively. In the great majority of cases, there was a good correlation between the expression of these three proteins, but no significant association with histological classification or TNM stage. Thus, E-cadherin-complex redistribution cannot be considered a prognostic marker in NET of the lung. Of particular interest was the frequent focal beta-catenin nuclear immunostaining (55.5% in total), which was also unrelated to histological type or TNM stage. However, this study failed to detect any mutation in exon 3 of the beta-catenin gene, in the APC gene or in the E-cadherin gene. These data suggest another mechanism of regulation of beta-catenin in these tumours., (Copyright 2001 John Wiley & Sons, Ltd.)

Wnt growth factors regulate a variety of developmental processes by altering specific gene expression patterns. In vertebrates beta-catenin acts as transcriptional activator, which is needed to overcome target gene repression by Groucho/TLE proteins, and to permit promoter activation as the final consequence of Wnt signaling. However, the molecular mechanisms of transcriptional activation by beta-catenin are only poorly understood. Here we demonstrate that the closely related acetyltransferases p300 and CBP potentiate beta-catenin-mediated activation of the siamois promoter, a known Wnt target. beta-catenin and p300 also synergize to stimulate a synthetic reporter gene construct, whereas activation of the cyclin D1 promoter by beta-catenin is refractory to p300 stimulation. Axis formation and activation of the beta-catenin target genes siamois and Xnr-3 in Xenopus embryos are sensitive to the E1A oncoprotein, a known inhibitor of p300/CBP. The C-terminus of beta-catenin interacts directly with a region overlapping the CH-3 domain of p300. p300 could participate in alleviating promoter repression imposed by chromatin structure and in recruiting the basal transcription machinery to promoters of particular Wnt target genes.

Aggressive fibromatosis (also called desmoid tumor) occurs as a sporadic lesion or as part of Familial Adenomatous Polyposis, which is caused by germ line mutations in the Adenomatous polyposis Coli (APC) gene. APC is involved in the regulation of the cellular level of beta-catenin, which is a mediator in Wnt signaling. Mutational analysis of the beta-catenin and APC genes was performed in 42 sporadic aggressive fibromatoses. Nine tumors had mutations in APC, and 22 had a point mutation in beta-catenin at either codon 45 or codon 41 (producing a stabilized beta-catenin protein product). Immunohistochemistry showed an elevated beta-catenin protein level in all tumors, regardless of mutational status. Beta-catenin localized to the nucleus, and was not tyrosine phosphorylated in the six tumors in which this was tested. The demonstration of mutations in two mediators in the Wnt-APC-beta-catenin pathway implicates beta-catenin stabilization as the key factor in the pathogenesis of aggressive fibromatosis. This is the first demonstration of somatic beta-catenin mutations in a locally invasive, but non metastatic lesion composed of spindle cells, illustrating the importance of beta-catenin stabilization in a variety of cell types and neoplastic processes. Moreover, this tumor has one of the highest reported frequencies of beta-catenin mutations of any tumor type.

A new human cDNA was cloned and termed alpha-catulin, based on sequence similarity with both alpha-CATenins and vincULIN. The mRNA is present ubiquitously, although low expression levels are found in neural tissues. The genomic organization of the alpha-catulin gene CTNNAL1 is closely related to that of the alphaE-catenin gene CTNNA1, but not at all to that of the vinculin gene. Alternative splicing of the last exon generates a frameshift, resulting in a truncated protein with a new carboxy-terminus.

The E-cadherin/catenin protein complex regulates the functional integrity of epithelia by mediating specific intercellular adhesion, Defects in the transmembrane E-cadherin protein play an important role in several human cancer types. E-cadherin-inactivating mutations were mainly found in sporadic lobular breast carcinoma and in both familial and sporadic diffuse gastric carcinoma. Armadillo proteins such as beta-catenin and p120ctn are complexed to the cytoplasmic tail of E-cadherin, whereas the vinculin-related alphaE-catenin protein forms a link to the actin cytoskeleton. The latter shows inactivating deletions in various tumor cell lines. Apparently, both E-cadherin and alphaE-catenin serve as tumor suppressor and invasion suppressor molecules. On the other hand, protein-stabilizing oncogenic mutations of beta-catenin were found at high frequency in particular human tumor types. Mutated beta-catenin protein is imported into the nucleus, and its binding to LEF/TCF transcription factors modulates transcription of intriguing target genes. Also p120ctn was recently found to arrive in the nucleus and to interact with a transcription factor. Furthermore, a wide variety of mechanisms have been described to regulate in a reversible way E-cadherin/catenin-mediated cell adhesion and differentiation. These phenomena appear to be crucial in human cancer development and progression.

Loss of serine or threonine phosphorylation sites from exon 3 of beta-catenin has been identified in approximately half of colorectal tumors which lack adenomatous polyposis coli (APC) mutations, but the overall contribution of beta-catenin mutations to sporadic colorectal tumorigenesis is unclear. We therefore used PCR to amplify and sequence exon 3 of beta-catenin from 202 sporadic colorectal tumors. Exon 3 beta-catenin mutations were identified in 6 of 48 small (< 1 cm) adenomas, 2 of 82 large (> or =1 cm) adenomas, and 1 of 72 invasive carcinomas. Eight of the nine mutations, including all of those in the small adenomas and the invasive cancer, involved loss of serine or threonine phosphorylation sites. The percentage of beta-catenin mutations in small adenomas (12.5%) was significantly greater than that in large adenomas (2.4%) and invasive cancers (1.4%; P = 0.05 and P = 0.02, respectively). We conclude that mutation of beta-catenin can be an early, perhaps initiating, event in colorectal tumorigenesis. Small adenomas with beta-catenin mutations do not appear to be as likely to progress to larger adenomas and invasive carcinomas as other adenomas, however, with the result that beta-catenin mutations are only rarely seen in invasive cancers. This suggests that APC and beta-catenin mutations are not functionally equivalent, and that the APC gene may have other tumor suppressor functions besides the degradation of beta-catenin.

The acquisition of invasiveness is a crucial step in the malignant progression of cancer. In cancers of the colon and of other organs the E-cadherin/catenin complex, which is implicated in homotypic cell-cell adhesion as well as in signal transduction, serves as a powerful inhibitor of invasion. We show here that one allele of the alphaE-catenin (CTNNA1) gene is mutated in the human colon cancer cell family HCT-8, which is identical to HCT-15, DLD-1 and HRT-18. Genetic instability, due to mutations in the HMSH6 (also called GTBP) mismatch repair gene, results in the spontaneous occurrence of invasive variants, all carrying either a mutation or exon skipping in the second alphaE-catenin allele. The alphaE-catenin gene is therefore, an invasion-suppressor gene in accordance with the two-hit model of Knudsen for tumour-suppressor genes.

Transition from an epithelioid (E) to a round (R) morphotype, in the human colon cancer cell line HCT-8, is associated with loss or truncation of alphaE-catenin and acquisition of invasiveness in organ culture. In E clones, like in parental HCT-8 cells, one allele of the alphaE-catenin gene (CTNNA1) is mutated. HCT-8 cells have also a 'Microsatelite Instability-High' (MSI-H) phenotype presumably due to a mutated hMSH6 gene. Fusion of E type cells doubles the wild type CTNNA1 alleles and prevents the loss of alphaE-catenin. Introduction of an extra chromosome 2, carrying a wild type hMSH6 gene, restores post-replicative mismatch repair and also prevents the frequent inactivation of the remaining wild type CTNNA1 allele.

Four human colon cancer cell lines, HCT-8, HRT-18, DLD-1, and HCT-15, with an epithelioid morphotype reproducibly formed alpha-catenin-deficient round cells. Using DNA fingerprinting, we found that these four cell lines have an identical genetic background. Our finding strongly suggests a genetic background for the reproducible loss of alpha-catenin and the ensuing acquisition of invasiveness in all four cell lines.

The E-cadherin-catenin complex, by mediating intercellular adhesion, regulates the architectural integrity of epithelia. Down-regulation of its expression is thought to contribute to invasion of carcinoma cells. To investigate the involvement of the E-cadherin-catenin adhesion system in the progression of human bronchopulmonary carcinomas, we compared the immunohistochemical distribution of E-cadherin, alpha-catenin, and beta-catenin in four human bronchial cancer cell lines with different invasive abilities and in 44 primary bronchopulmonary tumors. Although invasive bronchial cell lines did not express E-cadherin and alpha-catenin, complete down-regulation of cadherin-catenin complex expression was a rare event in vivo in bronchopulmonary carcinomas. Nevertheless, a spotty and cytoplasmic pattern of E-cadherin and catenins was observed in 32 primary tumors, only in invasive tumor clusters. Immunoprecipitation experiments showed that this redistribution was not related to a disruption of cadherin-catenin interaction but to down-regulated tyrosine phosphorylation of E-cadherin. We conclude that loss of E-cadherin and/or catenins is not a prominent early event in the invasive progression of human bronchopulmonary carcinomas in vivo. The decreased tyrosine phosphorylation of E-cadherin may reflect a loss of functionality of the complex and implicates a major role in tumor invasion.

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.

The different proteins of the E-cadherin/catenin cell-cell adhesion complex are believed to play a predominant role in carcinogenesis. Aberrant expression of these proteins has been found in many different human carcinomas, indicating abnormal regulation. In general, inactivating mutations of the human E-cadherin gene are rare; they are, however, highly frequent in infiltrating lobular breast carcinomas and in diffuse gastric carcinomas. These mutations mostly occur in combination with loss of heterozygosity (LOH) of the wild-type allele. Mutations were found at very early non-invasive stages, thus associating E-cadherin mutations with loss of growth control and defining E-cadherin as a real tumour suppressor for these particular tumour types. Defects affecting both alleles of the alpha E-catenin gene have been found in different human carcinoma cell lines, resulting in the loss of E-cadherin-mediated cell-cell adhesion. Mutations of the beta-catenin gene in colon tumours and melanomas were found to result in an accumulation of the protein in the cytosol. Upon translocation to the nucleus, this beta-catenin enhances TCF/LEF-dependent transcriptional activity. This suggests that mutated beta-catenin can act as an oncogene in these particular tumour types. The multiple interaction partners of beta-catenin are known to be involved in signal transduction, actin organization, protein phosphorylation or transcriptional regulation. This makes this protein an intriguing alternative target for either activation or inactivation in human cancer types characterized by frequent E-cadherin or APC deficiencies.

Loss of expression of the intercellular adhesion molecule E-cadherin frequently occurs in invasive lobular breast carcinomas as a result of mutational inactivation. Expression patterns of E-cadherin and the molecules comprising the cytoplasmic complex of adherens junctions, alpha-, beta- and gamma-catenin, were studied in a series of 38 lobular breast carcinomas with known E-cadherin mutation status. The effect of loss of E-cadherin by mutational inactivation (or other mechanisms) on the expression of catenins was investigated. Complete loss of plasma membrane-associated E-cadherin expression was observed in 32 out of 38 invasive lobular carcinomas, for which in 21 cases a mutation was found in the extracellular domain of E-cadherin. In total, 15 frameshift mutations of small deletions or insertions, ranging from 1 to 41 bp, three non-sense mutations, and three splice mutations were identified. Mutations were scattered over the whole coding region and no hot spots could be detected. In all cases, simultaneous loss of E-cadherin and alpha- and beta-catenin expression was found; in 50 per cent of these cases, additional loss of gamma-catenin was observed. In six invasive lobular carcinomas, expression of both E-cadherin and catenins was retained. In none of these carcinomas was an E-cadherin mutation detected. Lobular carcinoma in situ adjacent to invasive lobular carcinoma showed simultaneous loss of E-cadherin and catenins in all the cases studied--remarkably, also, in four cases positive for E-cadherin and catenin expression in the invasive component. These results indicate that simultaneous loss of E-cadherin and alpha-, beta- and gamma-catenin may be an important step in the formation of lobular carcinoma in situ, as a precursor of invasive lobular breast cancer. Events additional to E-cadherin inactivation must be involved in the transition of lobular carcinoma in situ to invasive lobular carcinoma.

The cytoplasmic protein alpha-catenin plays a crucial role in E-cadherin mediated cell-cell adhesion by binding E-cadherin to the cytoskeleton via beta- or gamma-catenin and actin. Functional loss of one of these interacting components leads to decreased cell-cell adhesion, and therefore to loss of epithelial integrity. Northern analysis revealed two distinct alphaE-catenin transcripts in different cell lines, whereas apparently only one protein is expressed. Because of the biological importance of this protein we sought to molecularly characterize the differences between the two observed transcripts. cDNA cloning and sequence analysis revealed the earlier described 3.4 kb alphaE-catenin transcript and an alphaE-catenin transcript of approximately 3.8 kb. This larger transcript contains a 321 bp extension in the 3'UTR sequence, which probably arises as a result of alternative polyadenylation. Considering the presence of AU-rich sequences in the extension, it may be involved in mRNA stability.

Invasion is the cause of cancer malignancy. Invasion results from the cross-talk between cancer cells and host cells, building molecular invasion-promoter and invasion-suppressor complexes. The E-cadherin/catenin invasion-suppressor complex is regulated multifactorially, at multiple levels and sometimes in a reversible way. Mutations in the E-cadherin gene combined with loss of the wild type allele, causing irreversible downregulation, has been demonstrated only in a minority of human cancers. Posttranslational and reversible downregulation has been ascribed to tyrosine phosphorylation of beta-catenin. Phosphorylation is also implicated in transmembrane receptor signal transduction through the E-cadherin/catenin complex. E-cadherin interacts with E-cadherin on another cell through a dimeric adhesion zipper, involving the histidine-alanine-valine (HAV) sequence of the first extracellular domains. This is the major extracellular like of the E-cadherin/catenin complex, though not the only one. Intracellularly, the list of proteins that bind to or signal through the complex or through one or more of its elements is steadily growing. Extrinsic factors may influence the complex. At least in vitro, insulin-like growth factor-I, retinoic acid, tangeretin and tamoxifen were shown to upregulate the functions of the E-cadherin/catenin complex including inhibition of invasion.

Invasion is a major challenge for cancer therapy. Invasion or noninvasion results from the cross talk between cancer cells and host cells, building molecular invasion-promoter and invasion-suppressor complexes. The E-cadherin/catenin invasion-suppressor complex is attractive as a target for a putative antiinvasive therapy because of its multifactorial regulation at multiple levels and sometimes in a reversible way. Mutations in the E-cadherin gene combined with loss of the wild type allele causes irreversible downregulation in some human cancers. Posttranslational and reversible downregulation may occur by tyrosine phosphorylation of beta-catenin. Phosphorylation is implicated also in transmembrane receptor signal transduction through the E-cadherin/catenin complex. Homophilic interaction with E-cadherin on another cell through a dimeric adhesion zipper, involving the HAV sequence of the first extracellular domains, is the major extracellular link of the E-cadherin/catenin complex. Intracellularly, the list of proteins that bind to or signal through the complex or one or more of its elements is growing. In vitro, insulin-like growth factor-I, and tamoxifen may upregulate the functions of the E-cadherin/catenin complex and inhibit invasion, demonstrating that this complex may serve as a target for antiinvasive therapy.

The cytoplasmic beta-catenin protein is implicated in signal transduction and associates with both the cell-cell adhesion protein E-cadherin and the tumor suppressor gene product APC. We determined the primary structure of the human beta-catenin gene (CTNNB1) by analysis of cDNA and genomic clones. The size of the complete gene was determined to be 23.2 kb. Restriction mapping and partial sequence analysis revealed 16 exons. All splice donor and acceptor sites were conformable to the GT/AG rule. The exon size ranged from 61 to 790 bp. Half of the introns were smaller than 550 bp, with the smallest being 84 bp and the longest being 6700 bp. The intron-exon boundaries did not coincide either with conserved sites in the 12 armadillo repeat sequences of beta-catenin or with intron-exon boundaries in the armadillo gene of Drosophila. A major site for transcription initiation was identified as an A residue 214 nucleotides upstream of the ATG initiation codon. The resulting transcript is 3362 nucleotides long. Compared to the previously published mRNA sequence, additional residues were identified, 16 at the 5' end and 766 at the 3' end of the mRNA. An alternative splice acceptor site within exon 16 reduced the 3' UTR sequence by 159 bp. Polymerase chain reaction on cDNA from 14 human cell lines demonstrated the general occurrence of both splice variants. The 5'-flanking region is highly GC-rich and lacks a CCAAT box, but contains a TATA box and potential binding sites for several transcription factors, such as NF kappa B, SP1, AP2, and EGR1. Both a 437-bp fragment and a 6-kb fragment, containing about 4.7 kb of the 5'-flanking region in addition to the noncoding exon 1 and 1 kb of intron 1, showed clear promoter activity when these fragments were linked to a secreted alkaline phosphatase reporter gene and transfected into a mouse epithelial cell line.

Loss of epithelioid organization in carcinoma cell lines has been related to invasiveness and poor differentiation of tumors. We investigated the invasion in vitro of various human colon cancer cell lines. Most cell lines were noninvasive into chick heart fragments, and this correlated with an epithelioid morphotype. Only cell lines COLO320DM, SW620, and variants of HCT-8 and DLD-1 were invasive and nonepithelioid. We examined in these cell lines whether invasiveness was related to changes in the structure and function of the E-cadherin/catenin complex. E-cadherin functions as an invasion suppressor and as a cell-cell adhesion molecule when linked to the cytoskeleton via alpha-catenin plus beta- or gamma-catenin. All noninvasive cell lines showed E-cadherin linked to these catenins. The E-cadherin-dependent cell-cell adhesion function in these cell lines was demonstrated by two assays in vitro. It was interesting that all invasive cell lines showed a dysfunctional E-cadherin/catenin complex. COLO320DM, SW480, and SW620 cells were defective in E-cadherin expression, whereas the invasive variants of HCT-8 and DLD-1 lacked the alpha-catenin protein. From clonal epithelioid HCT-8 cultures with functional E-cadherin/catenin complexes, we subcloned, repeatedly, round cell variants that were again invasive and expressed no alpha-catenin protein. Our data suggest that reproducible transformations toward a more invasive phenotype in HCT-8 cells are associated with down-regulation of alpha-catenin. The mechanisms of this transformation and the level of alpha-catenin down-regulation are currently investigated.

A pseudogene (CTNNAP1) for the human alpha E-catenin gene was isolated from a human genomic phage library. The pseudogene sequence shows 90% similarity to the alpha E-catenin mRNA at the nucleotide level. Thirty-eight stop codons in all three reading frames and multiple other mutations were found, indicating that the pseudogene does not encode a functional protein. No introns were found in the region corresponding to the open reading frame of the alpha E-catenin cDNA, and two direct repeats flank this same region. Hence, the pseudogene can be classified as a processed pseudogene. Polymerase chain reaction with pseudogene-specific primers on genomic DNA and cDNA from human cell lines and healthy blood donors demonstrated the general occurrence of the pseudogene and the lack of its transcription. By fluorescence in situ hybridization the pseudogene was mapped to human chromosome 5q22 and the alpha E-catenin gene to the formerly disputed locus 5q31. This is the first report of a pseudogene for a member of the cadherin-catenin cell-cell adhesion complex.

The human beta-catenin gene (CTNNB1) has been localized to 3p22-->p21.3 by fluorescence in situ hybridization. Recent studies have suggested the presence of one or more tumor suppressor genes on the short arm of chromosome 3. This raises the possibility that CTNNB1, for which important features are already known, is involved in tumor progression.

Loss of histotypic organization of epithelial cells is a common feature in normal development as well as in the invasion of carcinomas. Here we show that the v-src oncogene is a potent effector of epithelial differentiation and invasiveness. MDCK epithelial cells transformed with a temperature-sensitive mutant of v-src exhibit a strictly epithelial phenotype at the nonpermissive temperature for pp60v-src activity (40.5 degrees C) but rapidly loose cell-to-cell contacts and acquire a fibroblast-like morphology after culture at the permissive temperature (35 degrees C). Furthermore, the invasiveness of the cells into collagen gels or into chick heart fragments was increased at the permissive temperature. The profound effects of v-src on intercellular adhesion were not linked to changes in the levels of expression of the epithelial cell adhesion molecule E-cadherin. Rather, we observed an increase in tyrosine phosphorylation of E-cadherin and, in particular, of the associated protein beta-catenin. These results suggest a mechanism by which v-src counteracts junctional assembly and thereby promotes invasiveness and dedifferentiation of epithelial cells through phosphorylation of the E-cadherin/catenin complex.