Cara J. Gottardi

Cadherins and cancer: how does cadherin dysfunction promote tumor progression?

It has long been recognized that the cell–cell adhesion receptor, E-cadherin, is an important determinant of tumor progression, serving as a suppressor of invasion and metastasis in many contexts. Yet how the loss of E-cadherin function promotes tumor progression is poorly understood. In this review, we focus on three potential underlying mechanisms: the capacity of E-cadherin to regulate β-catenin signaling in the canonical Wnt pathway; its potential to inhibit mitogenic signaling through growth factor receptors and the possible links between cadherins and the molecular determinants of epithelial polarity. Each of these potential mechanisms provides insights into the complexity that is likely responsible for the tumor-suppressive action of E-cadherin.

A single amino acid in E‐cadherin responsible for host specificity towards the human pathogen Listeria monocytogenes

Human E‐cadherin promotes entry of the bacterial pathogen Listeria monocytogenes into mammalian cells by interacting with internalin (InlA), a bacterial surface protein. Here we show that mouse E‐cadherin, although very similar to human E‐cadherin (85% identity), is not a receptor for internalin. By a series of domain‐swapping and mutagenesis experiments, we identify Pro16 of E‐cadherin as a residue critical for specificity: a Pro→Glu substitution in human E‐cadherin totally abrogates interaction, whereas a Glu→Pro substitution in mouse E‐cadherin results in a complete gain of function. A correlation between cell permissivity and the nature of residue 16 in E‐cadherins from several species is established. The location of this key specificity residue in a region of E‐cadherin not involved in cell–cell adhesion and the stringency of the interaction demonstrated here have important consequences not only for the understanding of internalin function but also for the choice of the animal model to be used to study human listeriosis: mouse, albeit previously widely used, and rat appear as inappropriate animal models to study all aspects of human listeriosis, as opposed to guinea‐pig, which now stands as a small animal of choice for future in vivo studies.

Distinct molecular forms of β-catenin are targeted to adhesive or transcriptional complexes

β-Catenin plays essential roles in both cell–cell adhesion and Wnt signal transduction, but what precisely controls β-catenin targeting to cadherin adhesive complexes, or T-cell factor (TCF)-transcriptional complexes is less well understood. We show that during Wnt signaling, a form of β-catenin is generated that binds TCF but not the cadherin cytoplasmic domain. The Wnt-stimulated, TCF-selective form is monomeric and is regulated by the COOH terminus of β-catenin, which selectively competes cadherin binding through an intramolecular fold-back mechanism. Phosphorylation of the cadherin reverses the TCF binding selectivity, suggesting another potential layer of regulation. In contrast, the main cadherin-binding form of β-catenin is a β-catenin–α-catenin dimer, indicating that there is a distinct molecular form of β-catenin that can interact with both the cadherin and α-catenin. We propose that participation of β-catenin in adhesion or Wnt signaling is dictated by the regulation of distinct molecular forms of β-catenin with different binding properties, rather than simple competition between cadherins and TCFs for a single constitutive form. This model explains how cells can control whether β-catenin is used independently in cell adhesion and nuclear signaling, or competitively so that the two processes are coordinated and interrelated.

Adhesion signaling: How β-catenin interacts with its partners

Abstract The multi-functional protein β-catenin plays essential roles in cell–cell adhesion and nuclear signaling. Elucidation of the structures of β-catenin complexes is beginning to clarify how β-catenin uses the same surface to bind its various partners, and provides insights into how these interactions might be regulated.

Molecular Components of the Adherens Junction

Adherens junctions serve to couple individual cells into various arrangements required for tissue structure and function. The central structural components of adherens junctions are transmembrane adhesion receptors, and their associated actin-binding/regulatory proteins. The molecular machineries that organize these adhesion receptor complexes into higher order junction structures, and the functional consequences of this junctional organization will be discussed.

Polycystin-1 C-terminal tail associates with β-catenin and inhibits canonical Wnt signaling

Polycystin-1 (PC1), the product of the PKD1 gene mutated in the majority of autosomal dominant polycystic kidney disease (ADPKD) cases, undergoes a cleavage resulting in the intracellular release of its C-terminal tail (CTT). Here, we demonstrate that the PC1 CTT co-localizes with and binds to beta-catenin in the nucleus. This interaction requires a nuclear localization motif present in the PC1 CTT as well as the N-terminal portion of beta-catenin. The PC1 CTT inhibits the ability of both beta-catenin and Wnt ligands to activate T-cell factor (TCF)-dependent gene transcription, a major effector of the canonical Wnt signaling pathway. The PC1 CTT may produce this effect by reducing the apparent affinity of the interaction between beta-catenin and the TCF protein. DNA microarray analysis reveals that the canonical Wnt signaling pathway is activated in ADPKD patient cysts. Our results suggest a novel mechanism through which PC1 cleavage may impact upon Wnt-dependent signaling and thereby modulate both developmental processes and cystogenesis.

Mitochondrial reactive oxygen species promote epidermal differentiation and hair follicle development

Skin development requires reactive oxygen species generated by mitochondria in keratinocytes. Building a Barrier Mitochondria are an important source of reactive oxygen species (ROS), which participate in diverse signaling pathways. To test the role of mitochondrially produced ROS in epidermal development, Hamanaka et al. generated mice with keratinocytes lacking mitochondrial transcription factor A (TFAM), which is required for transcription of genes encoded by mitochondrial DNA, including those that proteins required for ROS generation. The epidermis of these mice was abnormally thick, lacked hair, and showed defects in differentiation and barrier function, which likely contributed to perinatal death. Keratinocytes from these mice did not produce mitochondrial ROS and showed impaired Notch signaling, which is involved in epidermal differentiation, and β-catenin signaling, which is required for growth of hair follicles. Thus, signaling pathways involved in skin development rely on the production of ROS generated by mitochondria. Proper regulation of keratinocyte differentiation within the epidermis and follicular epithelium is essential for maintenance of epidermal barrier function and hair growth. The signaling intermediates that regulate the morphological and genetic changes associated with epidermal and follicular differentiation remain poorly understood. We tested the hypothesis that reactive oxygen species (ROS) generated by mitochondria are an important regulator of epidermal differentiation by generating mice with a keratinocyte-specific deficiency in mitochondrial transcription factor A (TFAM), which is required for the transcription of mitochondrial genes encoding electron transport chain subunits. Ablation of TFAM in keratinocytes impaired epidermal differentiation and hair follicle growth and resulted in death 2 weeks after birth. TFAM-deficient keratinocytes failed to generate mitochondria-derived ROS, a deficiency that prevented the transmission of Notch and β-catenin signals essential for epidermal differentiation and hair follicle development, respectively. In vitro keratinocyte differentiation was inhibited in the presence of antioxidants, and the decreased differentiation marker abundance in TFAM-deficient keratinocytes was partly rescued by application of exogenous hydrogen peroxide. These findings indicate that mitochondria-generated ROS are critical mediators of cellular differentiation and tissue morphogenesis.

Canonical Wnt signaling induces skin fibrosis and subcutaneous lipoatrophy: a novel mouse model for scleroderma?

OBJECTIVE Because aberrant Wnt signaling has been linked with systemic sclerosis (SSc) and pulmonary fibrosis, we sought to investigate the effect of Wnt-10b on skin homeostasis and differentiation in transgenic mice and in explanted mesenchymal cells. METHODS The expression of Wnt-10b in patients with SSc and in a mouse model of fibrosis was investigated. The skin phenotype and biochemical characteristics of Wnt-10b-transgenic mice were evaluated. The in vitro effects of ectopic Wnt-10b were examined in explanted skin fibroblasts and preadipocytes. RESULTS The expression of Wnt-10b was increased in lesional skin biopsy specimens from patients with SSc and in those obtained from mice with bleomycin-induced fibrosis. Transgenic mice expressing Wnt-10b showed progressive loss of subcutaneous adipose tissue accompanied by dermal fibrosis, increased collagen deposition, fibroblast activation, and myofibroblast accumulation. Wnt activity correlated with collagen gene expression in these biopsy specimens. Explanted skin fibroblasts from transgenic mice demonstrated persistent Wnt/β-catenin signaling and elevated collagen and α-smooth muscle actin gene expression. Wnt-10b infection of normal fibroblasts and preadipocytes resulted in blockade of adipogenesis and transforming growth factor β (TGFβ)-independent up-regulation of fibrotic gene expression. CONCLUSION SSc is associated with increased Wnt-10b expression in the skin. Ectopic Wnt-10b causes loss of subcutaneous adipose tissue and TGFβ-independent dermal fibrosis in transgenic mice. These findings suggest that Wnt-10b switches differentiation of mesenchymal cells toward myofibroblasts by inducing a fibrogenic transcriptional program while suppressing adipogenesis. Wnt-10b-transgenic mice represent a novel animal model for investigating Wnt signaling in the setting of fibrosis.

β-catenin signaling: a novel mediator of fibrosis and potential therapeutic target

Purpose of reviewThe Wnt/&bgr;-catenin signaling pathway plays a critical role in development and adult tissue homeostasis. Recent investigations implicate Wnt/&bgr;-catenin signaling in abnormal wound repair and fibrogenesis. The purpose of this review is to highlight recent key studies that support a role for Wnt/&bgr;-catenin signaling in fibrosis. Recent findingsStudies of patients with fibrotic diseases have demonstrated changes in components of the Wnt/&bgr;-catenin pathway. In animal models, perturbations in Wnt/&bgr;-catenin signaling appear to aggravate or ameliorate markers of injury and fibrosis in a variety of different tissues. Studies also suggest that fibroblasts from different tissue sources may have markedly divergent responses to Wnt/&bgr;-catenin signaling. Cross-talk between Wnt/&bgr;-catenin and transforming growth factor-&bgr; pathways is complex and context-dependent, and may promote fibrogenesis through coregulation of fibrogenic gene targets. High throughput screening has identified several novel chemical inhibitors of Wnt/&bgr;-catenin signaling that may be of therapeutic potential. SummaryWnt/&bgr;-catenin signaling appears important in normal wound healing and its sustained activation is associated with fibrogenesis. The mechanism by which Wnt/&bgr;-catenin signaling may modify the response to injury is cell-type and context-dependent. Better understanding of this signaling pathway may provide a promising new therapeutic approach for human fibrotic diseases.

Activity of the β-catenin phosphodestruction complex at cell-cell contacts is enhanced by cadherin-based adhesion

It is well established that cadherin protein levels impact canonical Wnt signaling through binding and sequestering β-catenin (β-cat) from T-cell factor family transcription factors. Whether changes in intercellular adhesion can affect β-cat signaling and the mechanism through which this occurs has remained unresolved. We show that axin, APC2, GSK-3β and N-terminally phosphorylated forms of β-cat can localize to cell–cell contacts in a complex that is molecularly distinct from the cadherin–catenin adhesive complex. Nonetheless, cadherins can promote the N-terminal phosphorylation of β-cat, and cell–cell adhesion increases the turnover of cytosolic β-cat. Together, these data suggest that cadherin-based cell–cell adhesion limits Wnt signals by promoting the activity of a junction-localized β-cat phosphodestruction complex, which may be relevant to tissue morphogenesis and cell fate decisions during development.