'WNT-er is coming': WNT signalling in chronic lung diseases

Abstract

Chronic lung diseases represent a major public health problem with only limited therapeutic options. An important unmet need is to identify compounds and drugs that target key molecular pathways involved in the pathogenesis of chronic lung diseases. Over the last decade, there has been extensive interest in investigating Wingless/integrase-1 (WNT) signalling pathways; and WNT signal alterations have been linked to pulmonary disease pathogenesis and progression. Here, we comprehensively review the cumulative evidence for WNT pathway alterations in chronic lung pathologies, including idiopathic pulmonary fibrosis, pulmonary arterial hypertension, asthma and COPD. While many studies have focused on the canonical WNT/β-catenin signalling pathway, recent reports highlight that non-canonical WNT signalling may also significantly contribute to chronic lung pathologies; these studies will be particularly featured in this review. We further discuss recent advances uncovering the role of WNT signalling early in life, the potential of pharmaceutically modulating WNT signalling pathways and highlight (pre)clinical studies describing promising new therapies for chronic lung diseases.

Keywords: Airway Epithelium; Asthma; Asthma Mechanisms; COPD Pharmacology; COPD ÀÜ Mechanisms; Cytokine Biology; Idiopathic pulmonary fibrosis.

Figure 1

Schematic representation of canonical WNT/β-catenin signalling. Left side: cytosolic β-catenin is rapidly degraded by the β-catenin destruction complex in the absence of extracellular WNT ligands. The core of the β-catenin destruction complex is composed of: adenomatous polyposis coli (APC), axin, casein kinase-1 (CK-1) and glycogen synthase kinase-3 (GSK-3). GSK-3 is the primary kinase involved in the degradation of β-catenin. Right side: an extracellular WNT ligand binds and activates Frizzled (FZD) and the low density lipoprotein receptor-related proteins 5 and 6 (LRP5/6), which results in the activation of and intercellular signalling cascade that leads to the inhibition of the β-catenin destruction complex. Hence, β-catenin can accumulate and translocate to the nucleus to induce gene transcription. In the nucleus β-catenin can associate with various transcriptional coactivators, including T cell factor (TCF) and lymphoid enhancer factor (LEF).

Figure 2

Schematic representation of signalling cascades involved in non-canonical WNT signalling. An extracellular WNT ligand binds to the Frizzled (FZD) receptor, which can subsequently activate a variety of downstream signalling cascades involved in gene transcription, intercellular actin organisation and/or inhibition of the transcriptional coactivator β-catenin. AC, adenylylcyclase; PKA, protein kinase A; CREB, cAMP responsive element binding protein; DVL, dishevelled; FZD, Frizzled receptor; JNK, c-Jun-N terminal kinase; PLC, phospolipase C; NF-AT, nuclear factor of activated T cells; PKC, protein kinase C; PXN, paxillin; MRLC, myosin regulatory light chain; RAP1, RAS-related protein 1; CAMKII, calcium/calmodulin-dependent kinase II; NLK, Nemo-like-kinase.

Figure 3

WNT signalling in idiopathic pulmonary fibrosis (IPF) pathogenesis. Increased pulmonary expression of WNT-1, WNT-7B, WNT-10B, Frizzled receptor (FZD)2 and FZD3 in individuals with IPF. Enhanced expression of transcriptionally active β-catenin in pulmonary epithelial cells (top), as a consequence of WNT-3A and/or TGF-β signalling. β-catenin signalling induces mRNA expression of inflammatory and remodelling markers (eg, IL-1β and WNT1-inducible signalling protein-1 (WISP1)) and regulates alveolar epithelial type II cells (ATII)-to-ATI-cell transdifferentiation, a process implicated in wound healing and tissue regeneration. The profibrotic action of WISP1 can be diminished by neutralising antibodies. In pulmonary fibroblasts (bottom), WNT-5B by activating FZD8, in conjunction with TGF-β signalling, causes upregulation of mRNA expression of other WNT signal components, extracellular matrix (ECM) components, and myofibroblast markers. Both the expression of ECM components and markers of myofibroblast differentiation are dependent on activation of transcriptionally active β-catenin. Accumulation of transcriptionally active β-catenin can be prevented by small molecule inhibitors of Tankyrases (eg, XAV939), whereas the interaction of β-catenin with specific transcription factors can be inhibited by ICG-001 (β-catenin/cAMP response element-binding protein binding protein (CBP)), PKF115–584 (β-catenin/TCF (T cell factor)) or IQ-1 (β-catenin/p300). WNT-5A, via a yet unknown FZD, induces proliferation and protects cells from oxidative-stress-induced apoptosis. Lipoprotein receptor-related protein 5 (LRP5) and TGF-β signalling are indispensable for activation of β-catenin signalling in response to a fibrotic insult. Moreover, LRP5 in macrophages contributes to disease progression. See main text for further details.

Figure 4

WNT signalling in asthma. In airway smooth muscle cells, WNT-5A activates Frizzled receptor (FZD)8, which together with TGF-β stimulation results in increased mRNA expression of extracellular matrix (ECM) components. In addition, WNT-5A, via a not further specified FZD, enhances actin polymerisation and contractile capacity of the smooth muscle cell. Eosinophil-driven airway inflammation stimulates the smooth muscle cells to increase WNT-5A expression. Increased expression of WNT-1 prevents dendritic cell-mediated activation of T cells, thereby attenuating airway hyper-responsiveness (AHR) and airway remodelling. WNT-11, via an unspecified FZD, conjointly with TGF-β stimulation causes upregulation of the contractile protein α-sm-actin. Activation of transcriptionally active β-catenin in response to WNT-7B, TGF-β and/or (aero)allergens results in augmented mRNA expression of ECM components and genes involved in cell proliferation. Similarly, ectopic expression of a non-degradable form of β-catenin (S33Y-β-catenin) enhances expression of ECM components. Transcriptional activity of β-catenin can be inhibited by PKF115–584 or ICG-001, whereas the canonical WNT target gene WNT1-inducible signalling protein-1 (WISP1) can be inhibited with neutralising antibodies. See main text for further details.

Figure 5

WNT signalling in COPD. Neutrophil elastase and cigarette smoke attenuate pulmonary expression of WNT-2, WNT-3A, WNT-10B, LRP6, FZD1, AXIN1, AXIN2, CTNNB1 (β-catenin), LEF1 and TCF4 in human and/or animal models of COPD. In bronchial epithelial cells (left side of dashed line), WNT-4, independently of β-catenin, induces expression of extracellular matrix (ECM) components and of genes involved in cell proliferation. WNT-5B together with TGF-β/SMAD signalling activates gene transcription of MMP2, MMP9 and FN. Additionally, TGF-β inactivates glycogen synthase kinase-3 (GSK-3)β via phosphorylation resulting in activation of β-catenin, which facilitates the epithelial-to-mesenchymal transition (EMT) process of bronchial epithelial cells. In alveolar epithelial cells (right side of dashed line), β-catenin is a critical regulator of ATII-to-ATI-cell transdifferentiation. WNT-5A, which is increased in individuals with COPD and secreted by pulmonary fibroblasts, acts a negative regulator of β-catenin signalling, thereby impairing endogenous tissue repair by alveolar epithelial cells. FAM13, a COPD susceptibility gene, together with GSK-3β contributes to the development of emphysema by enhanced targeting of β-catenin for proteasomal degradation in alveolar epithelial cells. Moreover, cigarette smoke inhibits β-catenin signalling and epithelial cell repair by reducing FZD4 expression (indicated in grey). Pharmacological reactivation of β-catenin signalling via GSK-3β inhibition (by eg, LiCl, CT99021 or SB216763) in experimental emphysema in vivo as well as patient-derived COPD tissue ex vivo results in epithelial cell activation and attenuated emphysema pathology. In pulmonary fibroblasts, expression of proinflammatory genes induced by IL-1β or Epidermal growth factor (EGF) is mediated in part by FZD8. Additionally, IL-1β and EGF induce the expression of FZD8 via a yet unidentified signalling cascade, whereas TGF-β-induced expression of FZD8 is dependent on SMAD signalling. Additionally, TGF-β induces the expression of WNT-5A and WNT-5B, which in turn induce expression of proinflammatory genes in a FZD2-dependent and/or FZD8-dependent manner. Activated WNT signalling together with TGF-β induces mRNA expression of WNT signalling components, ECM components and myofibroblast markers. β-Catenin is required for the expression of ECM components and myofibroblast differentiation. See main text for further details.