Date Published: March 4, 2008
Publisher: Public Library of Science
Author(s): Moisés Selman, Annie Pardo, Naftali Kaminski
Abstract: The authors discuss evidence suggesting that embryonic signaling pathways involved in epithelium/mesenchymal communication and epithelial cell plasticity may be aberrantly switched on in idiopathic pulmonary fibrosis.
Partial Text: Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and usually lethal disease of uncertain etiology . It has been proposed that IPF likely results from an aberrant activation of alveolar epithelial cells after injury that provoke the migration, proliferation, and activation of mesenchymal cells with the formation of fibroblastic/myofibroblastic foci, leading to the exaggerated accumulation of extracellular matrix with the irreversible destruction of the lung parenchyma [2–4]. The molecular mechanisms that determine the persistent nature of IPF are poorly understood. While aberrant activation of developmental pathways that are usually suppressed in adult tissues is often noticed in cancer, it is only rarely observed in non-malignant diseases. In the following pages we present evidence from gene expression studies and animal models of disease that suggest that IPF is characterized by aberrant activation of developmental pathways (Box 1).
Microarray analysis identified an IPF-specific gene expression signature characterized by the up-regulation of genes indicative of an active tissue remodeling program, including extracellular matrix and a large number of myofibroblast/smooth muscle cell–associated and epithelial cell–related genes [3,5]. Recently, we have reanalyzed previously published datasets [3,5,6] using analytical approaches that allow global and unbiased mapping of the functional themes that characterize IPF lungs in comparison to controls or to other interstitial lung disease. The analyses revealed that IPF lungs were significantly enriched with genes associated with lung development . The up-regulated development-relevant genes included several members of transcription factor families such as the Sry-related high mobility group box and forkhead box, and genes related to the Wnt/β-catenin pathway [6,7]. (For complete datasets see http://www.dom.pitt.edu/paccm/Genomics/data.htm).
Epithelial cells are motile and can migrate away from their nearest neighbors . However, under normal conditions they do not detach and move away from the epithelial layer. This arrangement can be disturbed by a process known as epithelial–mesenchymal transition (EMT). In the process of EMT, epithelial cells lose many of their epithelial characteristics and obtain properties that are distinctive of mesenchymal cells . They become migratory, down-regulate the expression of cell adhesion molecules, primarily E-cadherin, lose their apical–basal polarity, and express mesenchymal molecules such as fibronectin and N-cadherin [11,12]. EMT is a key process in embryogenesis, where it leads to the formation of a migratory mesenchyme that progresses along the primitive streak and populates new areas of the embryo that will develop into mesoderm and endoderm .
An EMT-like process has been reported in cancer progression and metastasis, and in fibrotic disorders [13–23]. Recently, EMT was also observed in lung fibrosis by two groups that noticed numerous cells co-expressing epithelial and mesenchymal markers (thyroid transcription factor-1/α-smooth muscle actin  or surfactant protein C/N-cadherin ) within the expanded interstitium in IPF lungs. Moreover, using a triple transgenic mouse reporter system, Kim et al. demonstrated that EMT plays an important role during lung fibrogenesis and may be more widespread than previously thought .
Bone morphogenetic proteins (BMPs) and TGF-β belong to a superfamily of multifunctional cytokines that includes different isoforms with highly specific functions including wound healing, extracellular matrix remodeling, and the control of epithelial– mesenchymal interactions during embryogenesis [29,30]. Importantly, BMPs antagonize the effects of TGF-β regarding EMT and induce the inverse process of mesenchymal-to-epithelial transition . In tubular epithelial cells, BMP-7 reverses EMT by directly counteracting TGF-β-induced Smad-dependent cell signaling . In kidney fibrosis, this antagonism may lead to regeneration of injured tissues, suggesting that reversal of EMT may have therapeutic advantages and that fibrosis may be reversible . BMP-2 is decreased and BMP-4 is increased in IPF lungs, compared with controls (Table 1). More importantly, gremlin, the main BMP antagonist that modulates early limb outgrowth and patterning in the mouse embryo , is increased in IPF lungs . Gremlin is also found in human diabetic nephropathy, where it colocalizes with TGF-β . TGF-β induces gremlin expression in association with EMT in lung epithelial cells . Taken together, these data suggest that increased TGF-β expression, decreased BMP-2 expression, and active BMP inhibition by gremlin create an EMT-favoring environment in IPF lungs (Figure 1).
Wnts comprise a large family of secreted glycoproteins that activate multiple distinct types of intracellular signaling pathways through canonical and noncanonical Wnt pathways . Wnt signaling regulates a wide range of developmental processes, and its aberrant activation can lead to disease [41,42]. Canonical Wnt signaling inhibits the phosphorylation and degradation of β-catenin, allowing its translocation into the nucleus and its interaction with the high mobility group domain–containing, DNA-binding proteins (including the previously mentioned LEF-1) to regulate target gene expression [40–42]. β-catenin influences epithelial cell differentiation in the lung, and is required for the normal differentiation of the bronchiolar and alveolar epithelium .
Using gene expression microarrays, we demonstrated up-regulation of several members of the Wnt signaling pathway in IPF lungs, compared either with normal lungs or other interstitial lung diseases [3,5] (Table 1). For example, WISP-1 and the secreted frizzled-related protein 2 are increased in IPF compared with hypersensitivity pneumonitis . Several other Wnt pathway-related genes are also overexpressed in IPF lungs compared to normal controls (Table 1; dataset used in ; Gene Expression Omnibus database serial accession number GSE2052). The overall balance of the Wnt pathway genes overexpressed in IPF lungs seems to favor activation of the canonic pathway (Figure 2). To date, there is only one study that directly demonstrated aberrant nuclear localization of β-catenin in bronchiolar/alveolar epithelial cells and in fibroblasts from the fibroblastic foci in IPF lungs . Activation of β-catenin in epithelial cells is also indirectly corroborated by the overexpression of downstream genes such as MMP-7 and osteopontin [5,6], and may also be related to EMT.
A key process in the development of IPF is the formation of the fibroblastic focus. It has been suggested that these foci represent discrete isolated foci of fibroblasts/myofibroblasts. However, using three-dimensional reconstruction of the IPF lungs, other studies have suggested that fibroblast foci are the leading edge of a complex reticulum that is highly interconnected extending from the pleura into the underlying parenchyma .
Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) is a phosphatidylinositol phosphate phosphatase that is frequently deleted or inactivated in human cancers . PTEN is critical in development, and PTEN-null embryos die early during embryogenesis [66,67].
Sonic (Shh), Indian (Ihh), and Desert (Dhh) are the hedgehog family members in mammals. Shh is expressed in the developing lung epithelium, and its primary receptor, Patched-1 (Ptc), is found in mesenchymal cells. In vitro and in vivo studies have shown that Shh increases the proliferation of lung mesenchymal cells, up-regulating the expression of smooth muscle actin and myosin .
Identification and targeting of these abnormal mediators/pathways will eventually allow the development of therapeutic agents to control and hopefully achieve regression of IPF remodeling. Strategies designed to target the fibrogenic action of TGF-β with agents blocking its signaling pathway, such as the BMP-7, have led to repair of severely damaged renal tubular epithelial cells and to improvement of renal function and survival in mice with nephrotoxic serum nephritis . Along the same line of thought, hepatocyte growth factor, which among other functions inhibits TGF-β-induced EMT, may also be a useful therapeutic approach. It was recently shown that in vivo hepatocyte growth factor gene transfer reduced bleomycin-induced pulmonary fibrosis, improving alveolar epithelial repair, and importantly, decreasing TGF-β1 expression .
Many pathways that play an essential role during embryological development are inactivated later in life, although some of them may be transiently expressed during adult repair. Aberrant activation of these pathways during adult homeostasis leads to pathological events resulting in cancer, but may also be associated with the development of idiopathic pulmonary fibrosis. Dysfunctional activation of embryological pathways regularly repressed in the adult life may explain the persistent nature of the disease. Although some progress into unraveling the pathogenic mechanisms involved in IPF has been made, many open questions remain, and virtually no effective treatment is currently available. Designing and implementing interventions that target these embryological pathways may be required to develop novel anti-IPF therapies and to significantly improve the outcome of IPF patients.