p120-catenin and β-catenin differentially regulate cadherin adhesive function

Vascular endothelial (VE)-cadherin, the major adherens junction adhesion molecule in endothelial cells, interacts with p120-catenin and β-catenin through its cytoplasmic tail. However, the specific functional contributions of the catenins to the establishment of strong adhesion are not fully understood. Here we use bioengineering approaches to identify the roles of cadherin–catenin interactions in promoting strong cellular adhesion and the ability of the cells to spread on an adhesive surface. Our results demonstrate that the domain of VE-cadherin that binds to β-catenin is required for the establishment of strong steady-state adhesion strength. Surprisingly, p120 binding to the cadherin tail had no effect on the strength of adhesion when the available adhesive area was limited. Instead, the binding of VE-cadherin to p120 regulates adhesive contact area in a Rac1-dependent manner. These findings reveal that p120 and β-catenin have distinct but complementary roles in strengthening cadherin-mediated adhesion.     

Wnt/β-catenin signaling and disease

The WNT signal transduction cascade controls myriad biological phenomena throughout development and adult life of all animals. In parallel, aberrant Wnt signaling underlies a wide range of pathologies in humans. In this Review, we provide an update of the core Wnt/β-catenin signaling pathway, discuss how its various components contribute to disease, and pose outstanding questions to be addressed in the future.     

Biphasic and dosage-dependent regulation of osteoclastogenesis by β-catenin

Wnt/β-catenin signaling is a critical regulator of skeletal physiology. However, previous studies have mainly focused on its roles in osteoblasts, while its specific function in osteoclasts is unknown. This is a clinically important question because neutralizing antibodies against Wnt antagonists are promising new drugs for bone diseases. Here, we show that in osteoclastogenesis, β-catenin is induced during the macrophage colony-stimulating factor (M-CSF)-mediated quiescence-to-proliferation switch but suppressed during the RANKL-mediated proliferation-to-differentiation switch. Genetically, β-catenin deletion blocks osteoclast precursor proliferation, while β-catenin constitutive activation sustains proliferation but prevents osteoclast differentiation, both causing osteopetrosis. In contrast, β-catenin heterozygosity enhances osteoclast differentiation, causing osteoporosis. Biochemically, Wnt activation attenuates whereas Wnt inhibition stimulates osteoclastogenesis. Mechanistically, β-catenin activation increases GATA2/Evi1 expression but abolishes RANKL-induced c-Jun phosphorylation. Therefore, β-catenin exerts a pivotal biphasic and dosage-dependent regulation of osteoclastogenesis. Importantly, these findings suggest that Wnt activation is a more effective treatment for skeletal fragility than previously recognized that confers dual anabolic and anti-catabolic benefits.     

δ-catenin affects the localization and stability of p120-catenin by competitively interacting with E-cadherin

E-cadherin is a member of the cadherin family of Ca²⁺-dependent cell-cell adhesion molecules. p120-Catenin and δ-catenin are known to bind to similar juxtamembrane regions of E-cadherin, and p120-catenin is known to stabilize E-cadherin. However, the function of competition between p120-catenin and δ-catenin for E-cadherin has not been fully explained. In this report, we show that cells overexpressing δ-catenin contain less p120-catenin than control cells at the cell-cell interface and that this causes the relocalization of p120-catenin from the plasma membrane to the cytosol. We show that successful binding by one to E-cadherin adversely affects the stability of the other.     

Absolute β-catenin concentrations in Wnt pathway-stimulated and non-stimulated cells

The intracellular level of the proto-oncoprotein β-catenin is a parameter for the activity of the Wnt pathway, which has been linked to carcinogenesis. The paper introduces a novel sandwich-based ELISA for the determination of the β-catenin concentration in lysates from cells or tissues. The advantages of the method were proven by determining β-catenin levels in cell lines and in cells after activation of the Wnt pathway. Analysis revealed high β-catenin concentrations in the cell lines HeLa, KB, HT1080, MCF-7, U-87 and U-373, which had not been described before. β-Catenin concentrations were compared in HEK293 and C57MG cells after activation of the Wnt pathway. The β-catenin concentrations increased by different factors depending on whether the Wnt pathway was activated by incubation with LiCl or with Wnt-3a-conditioned medium. This finding indicated that the β-catenin level depends on the way and level of Wnt pathway activation. The quantitative analysis of β-catenin in colorectal tumours revealed high β-catenin levels in tumours with truncating mutations in the APC gene.     

α-catenin

Downregulation or loss of α-catenin occurs in multiple human cancer types. The traditional view of α-catenin is that it is one of the core components of the E-cadherin-catenin complex and is required for maintaining the integrity of the intercellular adherens junction, a cell junction whose cytoplasmic face is linked to the actin cytoskeleton. Therefore, loss of α-catenin can result in loss of cell-cell adhesion, a common characteristic of cancer cells. There is an emerging recognition; however, that α-catenin also regulates multiple signaling pathways independent of adherens junctions. For instance, α-catenin functions as a tumor suppressor in E-cadherin-negative basal like breast cancer cells by inhibiting NF-κB signaling. In this perspective, we discuss the role and mechanisms of α-catenin in regulating several signaling pathways in cancer.     

Analysing the impact of nucleo-cytoplasmic shuttling of β-catenin and its antagonists APC,Axin and GSK3 on Wnt/β-catenin signalling

The canonical Wnt signalling pathway plays a critical role in development and disease. The key player of the pathway is β-catenin. Its activity is mainly regulated by the destruction complex consisting of APC, Axin and GSK3. In the nucleus, the complex formation of β-catenin and TCF initiates target gene expression. Our study provides a comprehensive analysis of the role of nucleo-cytoplasmic shuttling of APC, Axin, and GSK3 and the inactivation of β-catenin by the destruction complex in Wnt/β-catenin signalling.We address the following questions: Can nucleo-cytoplasmic shuttling of APC, Axin and GSK3 increase the [β-catenin/TCF] concentration? And, how is the [β-catenin/TCF] concentration influenced by phosphorylation and subsequent degradation of nuclear β-catenin?Based on experimental findings, we develop a compartmental model and conduct several simulation experiments. Our analysis reveals the following key findings: 1) nucleo-cytoplasmic shuttling of β-catenin and its antagonists can yield a spatial separation between the said proteins, which results in a breakdown of β-catenin degradation, followed by an accumulation of β-catenin and hence leads to an increase of the [β-catenin/TCF] concentration. Our results strongly suggest that Wnt signalling can benefit from nucleo-cytoplasmic shuttling of APC, Axin and GSK3, although they are in general β-catenin antagonising proteins. 2) The total robustness of the [β-catenin/TCF] output is closely linked to its absolute concentration levels. We demonstrate that the compartmental separation of β-catenin and the destruction complex does not only lead to a maximization, but additionally to an increased robustness of [β-catenin/TCF] signalling against perturbations in the cellular environment. 3) A nuclear accumulation of the destruction complex renders the pathway robust against fluctuations in Wnt signalling and against changes in the compartmental distribution of β-catenin. 4) Elucidating the impact of destruction complex inhibition, we show that the [β-catenin/TCF] concentration is more effectively enhanced by inhibition of the kinase GSK3 rather than the binding of β-catenin to the destruction complex.     

C-Src-mediated phosphorylation of δ-catenin increases its protein stability and the ability of inducing nuclear distribution of β-catenin

Although δ-catenin was first considered as a brain specific protein, strong evidence of δ-catenin overexpression in various cancers, including prostate cancer, has been accumulated. Phosphorylation of δ-catenin by Akt and GSK3β has been studied in various cell lines. However, tyrosine phosphorylation of δ-catenin in prostate cancer cells remains unknown. In the current study, we demonstrated that Src kinase itself phosphorylates δ-catenin on its tyrosine residues in prostate cancer cells and further illustrated that Y1073, Y1112 and Y1176 of δ-catenin are predominant sites responsible for tyrosine phosphorylation mediated by c-Src. Apart from c-Src, other Src family kinases, including Fgr, Fyn and Lyn, can also phosphorylate δ-catenin. We also found that c-Src-mediated Tyr-phosphorylation of δ-catenin increases its stability via decreasing its affinity to GSK3β and enhances its ability of inducing nuclear distribution of β-catenin through interrupting the integrity of the E-cadherin. Taken together, these results indicate that c-Src can enhance the oncogenic function of δ-catenin in prostate cancer cells.     

β-catenin and early development in the gastropod, Crepidula fornicata

This study describes the early expression and function of β-catenin in the gastropod, Crepidula fornicata. In other bilaterians β-catenin functions in cell adhesion, gastrulation, and cell signaling, which is related to the establishment of the dorso-ventral axis and mesendoderm. Here, we studied the distribution of β-catenin mRNA and protein in C. fornicata via whole mount in situ hybridization and by expressing GFP-tagged β-catenin in vivo. During early cleavage, β-catenin mRNA and protein appear to be broadly localized to all cells in the early embryo. The mRNA tends to be concentrated at inter-phase centrosomes in these cells. At later stages, the mRNA is predominantly in the vegetal macromeres, and subsequently in the rudiment of the hindgut, stomodeum, and velar lobes. Expression of full-length GFP-tagged protein suggests that there is no active mechanism to degrade β-catenin within cells of the early embryos prior to the 25-cell stage. However, by the second day of development, when the fourth quartet micromeres have formed, β-catenin becomes selectively stabilized in the progeny of the 4d mesentoblast (e.g., ML and MR and their daughters) and is missing from most other blastomeres, including vegetal macromeres. Over the next 2 days of development, during subsequent divisions of 4d, β-catenin protein becomes progressively degraded, along the proximo-distal axes, within the progeny of the paired mesendodermal bands. The cells located at the tips of the mesodermal bands (2?mL2 and 2?mR2) are the last to contain this protein, which is no longer detected after 4 days of development. In animals like C. fornicata, which undergo a spiral cleavage program (e.g., molluscs, annelids, nemerteans, and polyclad flatworms), the mesentoblast or 4d cell represents the progenitor of endomesoderm (forming hindgut, internal and external kidneys, and various muscles). Therefore, the selective stabilization of β-catenin in the progeny of 4d in C. fornicata is consistent with arguments that a basic, ancestral role of β-catenin lies in the formation of endomesodermal fates. Experiments using a truncated β-catenin clone show that the regions located in the C-terminus, distal to the 11th armadillo repeat, are required for normal stabilization/degradation of β-catenin protein within the embryo. Microinjection of translation blocking β-catenin morpholinos into zygotes led to the down-regulation of β-catenin expression. This resulted in the subsequent failure of gastrulation, but did not interfere with the formation and early cleavage of 4d, although there were no discernable differentiated cell fates in these defective embryos. These results are compared with those obtained in other metazoans.     

Compartmentalized CDK2 is connected with SHP-1 and β-catenin and regulates insulin internalization

The cyclin-dependant kinase Cdk2 is compartmentalized in endosomes but its role is poorly understood. Here we show that Cdk2 present in hepatic endosome fractions is strictly located in a Triton X-100-resistant environment. The endosomal Cdk2 was found to be associated with the protein tyrosine phosphatase SHP-1, a regulator of insulin clearance, and the actin anchor β-catenin, a known substrate for both Cdk2 and SHP-1. In the plasma membranes and endosome fractions, β-catenin is associated with CEACAM1, also known as regulator of insulin clearance. We show that β-catenin, not CEACAM1, is a substrate for Cdk2. Partial down-modulation of Cdk2 in HEK293 cells increased the rate of insulin internalization. These findings reveal that Cdk2 functions, at least in part, via a Cdk2/SHP-1/β-catenin/CEACAM1 axis, and show for the first time that Cdk2 has the capacity to regulate insulin internalization.     

The role of pleiotrophin and β-catenin in fetal lung development

Mammalian lung development is a complex biological process, which is temporally and spatially regulated by growth factors, hormones, and extracellular matrix proteins. Abnormal changes of these molecules often lead to impaired lung development, and thus pulmonary diseases. Epithelial-mesenchymal interactions are crucial for fetal lung development. This paper reviews two interconnected pathways, pleiotrophin and Wnt/β-catenin, which are involved in fibroblast and epithelial cell communication during fetal lung development.