Wnt signaling in lung organogenesis

Reporter transgene, knockout, and misexpression studies support the notion that Wnt/β-catenin signaling regulates aspects of branching morphogenesis, regional specialization of the epithelium and mesenchyme, and establishment of progenitor cell pools. As demonstrated for other foregut endoderm-derived organs, β-catenin and the Wnt/β-catenin signaling pathway contribute to control of cellular proliferation, differentiation and migration. However, the contribution of Wnt/β-catenin signaling to these processes is shaped by other signals impinging on target tissues. In this review, we will concentrate on roles for Wnt/β-catenin in respiratory system development, including segregation of the conducting airway and alveolar compartments, specialization of the mesencyme, and establishment of tracheal asymmetries and tracheal glands.     

Functional Wnt signaling is increased in idiopathic pulmonary fibrosis

Background

Idiopathic pulmonary fibrosis (IPF) is a fatal lung disease, characterized by distorted lung architecture and loss of respiratory function. Alveolar epithelial cell injury and hyperplasia, enhanced extracellular matrix deposition, and (myo)fibroblast activation are features of IPF. Wnt/β-catenin signaling has been shown to determine epithelial cell fate during development. As aberrant reactivation of developmental signaling pathways has been suggested to contribute to IPF pathogenesis, we hypothesized that Wnt/β-catenin signaling is activated in epithelial cells in IPF. Thus, we quantified and localized the expression and activity of the Wnt/β-catenin pathway in IPF.

Methodology/Principal Findings

The expression of Wnt1, 3a, 7b, and 10b, the Wnt receptors Fzd1-4, Lrp5-6, as well as the intracellular signal transducers Gsk-3β, β-catenin, Tcf1, 3, 4, and Lef1 was analyzed in IPF and transplant donor lungs by quantitative real-time (q)RT-PCR. Wnt1, 7b and 10b, Fzd2 and 3, β-catenin, and Lef1 expression was significantly increased in IPF. Immunohistochemical analysis localized Wnt1, Wnt3a, β-catenin, and Gsk-3β expression largely to alveolar and bronchial epithelium. This was confirmed by qRT-PCR of primary alveolar epithelial type II (ATII) cells, demonstrating a significant increase of Wnt signaling in ATII cells derived from IPF patients. In addition, Western blot analysis of phospho-Gsk-3β, phospho-Lrp6, and β-catenin, and qRT-PCR of the Wnt target genes cyclin D1, Mmp 7, or Fibronectin 1 demonstrated increased functional Wnt/β-catenin signaling in IPF compared with controls. Functional in vitro studies further revealed that Wnt ligands induced lung epithelial cell proliferation and (myo)fibroblast activation and collagen synthesis.

Conclusions/Significance

Our study demonstrates that the Wnt/β-catenin pathway is expressed and operative in adult lung epithelium. Increased Wnt/β-catenin signaling may be involved in epithelial cell injury and hyperplasia, as well as impaired epithelial-mesenchymal cross-talk in IPF. Thus, modification of Wnt signaling may represent a therapeutic option in IPF.     

Phytochemicals Attenuating Aberrant Activation of ��-Catenin in Cancer Cells

Phytochemicals are a rich source of chemoprevention agents but their effects on modulating the Wnt/β-catenin signaling pathway have remained largely uninvestigated. Aberrantly activated Wnt signaling can result in the abnormal stabilization of β-catenin, a key causative step in a broad spectrum of cancers. Here we report the modulation of lithium chloride-activated canonical Wnt/β-catenin signaling by phytochemicals that have antioxidant, anti-inflammatory or chemopreventive properties. The compounds were first screened with a cervical cancer-derived stable Wnt signaling reporter HeLa cell line. Positive hits were subsequently evaluated for β-catenin degradation, suppression of β-catenin nuclear localization and down-regulation of downstream oncogenic targets of Wnt/β-catenin pathway. Our study shows a novel degradation path of β-catenin protein in HeLa cells by Avenanthramide 2p (a polyphenol) and Triptolide (a diterpene triepoxide), respectively from oats and a Chinese medicinal plant. The findings present Avenanthramide 2p as a potential chemopreventive dietary compound that merits further study using in vivo models of cancers; they also provide a new perspective on the mechanism of action of Triptolide.     

Smad3 Prevents ��-Catenin Degradation and Facilitates ��-Catenin Nuclear Translocation in Chondrocytes

Our previous study demonstrated that transforming growth factor (TGF)-β activates β-catenin signaling through Smad3 interaction with β-catenin in chondrocytes. In the present studies, we further investigated the detailed molecular mechanism of the cross-talk between TGF-β/Smad3 and Wnt/β-catenin signaling pathways. We found that C-terminal Smad3 interacted with both the N-terminal region and the middle region of β-catenin protein in a TGF-β-dependent manner. Both Smad3 and Smad4 were required for the interaction with β-catenin and protected β-catenin from an ubiquitin-proteasome-dependent degradation. In addition, the formation of the Smad3-Smad4-β-catenin protein complex also mediated β-catenin nuclear translocation. This Smad3-mediated regulatory mechanism of β-catenin protein stability enhanced the activity of β-catenin to activate downstream target genes during chondrogenesis. Our findings demonstrate a novel mechanism between TGF-β and Wnt/β-catenin signaling pathways during chondrocyte development.     

��-Catenin/T-cell Factor Signaling Is Activated during Lung Injury and Promotes the Survival and Migration of Alveolar Epithelial Cells

The Wnt/β-catenin signaling cascade activates genes that allow cells to adopt particular identities throughout development. In adult self-renewing tissues like intestine and blood, activation of the Wnt pathway maintains a progenitor phenotype, whereas forced inhibition of this pathway promotes differentiation. In the lung alveolus, type 2 epithelial cells (AT2) have been described as progenitors for the type 1 cell (AT1), but whether AT2 progenitors use the same signaling mechanisms to control differentiation as rapidly renewing tissues is not known. We show that adult AT2 cells do not exhibit constitutive β-catenin signaling in vivo, using the AXIN2^+/LacZ reporter mouse, or after fresh isolation of an enriched population of AT2 cells. Rather, this pathway is activated in lungs subjected to bleomycin-induced injury, as well as upon placement of AT2 cells in culture. Forced inhibition of β-catenin/T-cell factor signaling in AT2 cultures leads to increased cell death. Cells that survive show reduced migration after wounding and reduced expression of AT1 cell markers (T1α and RAGE). These results suggest that AT2 cells may function as facultative progenitors, where activation of Wnt/β-catenin signaling during lung injury promotes alveolar epithelial survival, migration, and differentiation toward an AT1-like phenotype.     

Wnt signal transduction pathways

The Wnt signaling pathway is an ancient and evolutionarily conserved pathway that regulates crucial aspects of cell fate determination, cell migration, cell polarity, neural patterning and organogenesis during embryonic development. The Wnts are secreted glycoproteins and comprise a large family of nineteen proteins in humans hinting to a daunting complexity of signaling regulation, function and biological output. To date major signaling branches downstream of the Fz receptor have been identified including a canonical or Wnt/β-catenin dependent pathway and the non-canonical or β-catenin-independent pathway which can be further divided into the Planar Cell Polarity and the Wnt/Ca²⁺ pathways, and these branches are being actively dissected at the molecular and biochemical levels. In this review, we will summarize the most recent advances in our understanding of these Wnt signaling pathways and the role of these pathways in regulating key events during embryonic patterning and morphogenesis.Key words: Wnt, frizzled, dishevelled, canonical, non-canonical, β-catenin, Planar Cell Polarity     

Exosome release of ��-catenin: a novel mechanism that antagonizes Wnt signaling

CD82 and CD9 are tetraspanin membrane proteins that can function as suppressors of tumor metastasis. Expression of CD9 and CD82 in transfected cells strongly suppresses β-catenin–mediated Wnt signaling activity and induces a significant decrease in β-catenin protein levels. Inhibition of Wnt/β-catenin signaling is independent of glycogen synthase kinase-3β and of the proteasome- and lysosome-mediated protein degradation pathways. CD82 and CD9 expression induces β-catenin export via exosomes, which is blocked by a sphingomyelinase inhibitor, GW4869. CD82 fails to induce exosome release of β-catenin in cells that express low levels of E-cadherin. Exosome release from dendritic cells generated from CD9 knockout mice is reduced compared with that from wild-type dendritic cells. These results suggest that CD82 and CD9 down-regulate the Wnt signaling pathway through the exosomal discharge of β-catenin. Thus, exosomal packaging and release of cytosolic proteins can modulate the activity of cellular signaling pathways.     

Wnt/��-catenin signaling in cerebral cortical development

The cerebral cortex is the multilayered sheet of neurons that underlies our highest cognitive abilities. Canonical Wnt/β-catenin signaling has well-known activities in tissue patterning in regulating rostral-caudal and medial-lateral patterning in the developing cortex. In addition, recent studies suggest that Wnt/β-catenin signaling also plays important roles in establishing the radial inside to outside organization of the cerebral cortex. Different Wnts, Wnt receptors and inhibitors are expressed in overlapping radial compartments of the cerebral cortex, and in vivo functional studies have provided evidence for Wnt/β-catenin regulation of neural precursor self-renewal, laminar fate determination and establishing or stabilizing the patterns of neuronal communication of cortical neurons. Wnt/β-catenin alterations have been observed in human brain tumors, and understanding its many diverse functions during normal neural development may provide greater insight into the mechanisms underlying the development and progression of neural tumors.Key words: cerebral cortex, neural stem cell, neural precursor, ventricular zone, laminar fate, regional specification, radial patterning     

Reconstitution of a Frizzled8��Wnt3a��LRP6 Signaling Complex Reveals Multiple Wnt and Dkk1 Binding Sites on LRP6

Wnt/β-catenin signaling is initiated at the cell surface by association of secreted Wnt with its receptors Frizzled (Fz) and low density lipoprotein receptor-related protein 5/6 (LRP5/6). The study of these molecular interactions has been a significant technical challenge because the proteins have been inaccessible in sufficient purity and quantity. In this report we describe insect cell expression and purification of soluble mouse Fz8 cysteine-rich domain and human LRP6 extracellular domain and show that they inhibit Wnt/β-catenin signaling in cellular assays. We determine the binding affinities of Wnts and Dickkopf 1 (Dkk1) to the relevant co-receptors and reconstitute in vitro the Fz8 CRD·Wnt3a·LRP6 signaling complex. Using purified fragments of LRP6, we further show that Wnt3a binds to a region including only the third and fourth β-propeller domains of LRP6 (E3E4). Surprisingly, we find that Wnt9b binds to a different part of the LRP6 extracellular domain, E1E2, and we demonstrate that Wnt3a and Wnt9b can bind to LRP6 simultaneously. Dkk1 binds to both E1E2 and E3E4 fragments and competes with both Wnt3a and Wnt9b for binding to LRP6. The existence of multiple, independent Wnt binding sites on the LRP6 co-receptor suggests new possibilities for the architecture of Wnt signaling complexes and a model for broad-spectrum inhibition of Wnt/β-catenin signaling by Dkk1.     

Wnt/��-catenin signaling in hepatic organogenesis

Wnt/β-catenin signaling has come to the forefront of liver biology in recent years. This pathway regulates key pathophysiological events inherent to the liver including development, regeneration and cancer, by dictating several biological processes such as proliferation, apoptosis, differentiation, adhesion, zonation and metabolism in various cells of the liver. This review will examine the studies that have uncovered the relevant roles of Wnt/β-catenin signaling during the process of liver development. We will discuss the potential roles of Wnt/β-catenin signaling during the phases of development, including competence, hepatic induction, expansion and morphogenesis. In addition, we will discuss the role of negative and positive regulation of this pathway and how the temporal expression of Wnt/β-catenin can direct key processes during hepatic development. We will also identify some of the major deficits in the current understanding of the role of Wnt/β-catenin signaling in liver development in order to provide a perspective for future studies. Thus, this review will provide a contextual overview of the role of Wnt/β-catenin signaling during hepatic organogenesis.Key words: liver development, liver cancer, liver regeneration, Wnt signaling, proliferation, differentiationThe Wnt/β-catenin pathway is an evolutionarily well-conserved pathway that has proven to be essential to normal cellular processes such as development, growth, survival, regeneration and self-renewal.^1–5 Its diverse functions also include the initiation and progression of cancer.⁶ In fact, one area in which this pathway has been extensively studied is in liver cancer.Mutations of Wnt/β-catenin pathway members in hepatocarcinogenesis are common. For example, 90–100% of hepatoblastomas contain mutations in adenomatous polyposis coli (APC), CTNNB1 and/or Axin1/2, which causes cytoplasmic and nuclear localization of β-catenin.^7–9 Axin1 and β-catenin mutations have also been identified in approximately 25% of hepatocellular carcinomas,^10–12 while overexpression of the frizzled-7 receptor¹³ and glycogen synthase kinase-3 (GSK-3) inactivation¹⁴ can also lead to aberrant β-catenin pathway activation. The dysregulation of this pathway in hepatic cancers makes it an attractive target for potential therapies, and experimental treatment in vivo has shown promising results. For example, inhibiting β-catenin expression by siRNA or R-Etodolac decreases proliferation and survival of human hepatoma cell lines.^15,16 Since cancer recapitulates development, determining the timing of β-catenin activation during hepatogenesis will help us to better understand the inappropriate activation of this pathway in hepatocarcinogenesis.Recent work has elucidated the role of β-catenin signaling in the liver, and has highlighted its essential role in liver health and disease.¹⁷ In addition, emerging evidence suggests that this pathway plays a key role in liver organogenesis.