The Fibroblast Growth Factor Pathway and Its Role in the Pathogenesis of Lung Disease

Introduction

F ibroblast growth factors (FGFs) comprise a number of structurally related polypeptide growth factors, with a diverse array of possible cellular functions. Their signals are transduced via receptors, and most FGFs have high affinity to a specific receptor(s). FGF 1 binds to most receptors, while FGF 7 binds to a splice variant of FGF receptor 2 (FGFR2). FGF 7, in particular, holds interest in understanding repair mechanisms of the lung, as it appears specific for epithelial cells (Werner and Grose 2003). FGF signaling is also a key regulatory pathway in the coordination of mesodermal, ectodermal, and endodermal organ development. The FGF pathway is an essential signaling pathway controlling angiogenesis, morphogenesis, and remodeling in the airway (Fig. 1).

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FIG. 1.

The fibroblast growth factor pathway. Reference: Infinity (modified from The Scripps Research Institute, La Jolla, CA).

FGF and the Growing Lung

The growing lung involves an intricate coordination of multiple pathways that control the development of airways, branching, and alveolarization. In a normal fetus, the first 23 generations of airway branching are completed between 5 and 25 weeks of gestation. Alveolarization commences at 20 weeks and continues for years after birth. The pulmonary vasculature develops in a branching pattern that follows the airways. FGF is required for airway branching and is essential for the formation of all the branches of the airway. In studies of mice with null mutations of FGFR2, the binding receptor for FGF 7, complete arrest of airway branching distal to the trachea occurs (Warburton and others 2000; Warburton and Bellusci 2004).

FGF regulates a select number of alveolar stem cells that resist hyperoxic apoptosis (Warburton and others 2001), and null mutations of FGFR3 and FGFR4 disrupt postnatal alveolar morphogenesis (Weinstein and others 1998).

FGF and Premature Lung Disease

Premature delivery is associated with underdevelopment of the alveoli, exposure to high levels of supplemental oxygen, and airway pressures, which result in alveolar remodeling. After a time period of exposure to the treatment of premature lung disease, including mechanical ventilation and prolonged supplemental oxygen exposure, bronchopulmonary dysplasia (BPD) can develop. In some studies, alveolar type 2 epithelial cells are shown to repair damaged airways with rapid migration and proliferation responses, and they may be central in controlling the extent of BPD (Warburton and others 2001).

In one study of 29 preterm infants under 34 weeks of gestational age, and between 500 and 2,000 g birth weight, tracheal aspirates showed elevated levels of basic FGF (bFGF) in infants who died or later developed BPD. In contrast, vascular endothelial growth factor (VEGF) and endothelin-1 levels did not correlate with these outcomes (Ambalavanan and Novak 2003). The bFGF levels were correlated to the severity of cellular injury and apoptosis in these infants. These findings along with those of animal models indicate that FGF plays a role in the remodeling of the developing neonatal airway resulting in BPD. In models of acute hyperoxic injury, FGF 7 decreased lung injury and improved survival. In studies in which FGF signaling was inhibited in a model of hyperoxic lung injury, inflammatory cytokines [interleukin-1β (IL-1β) and IL-6] were increased, and surfactant proteins A and B were decreased in the affected lung. Restoration of FGF signaling transcription factor thyroid transcription factor 1 reversed the cellular injury. FGF signaling and its associated transcription factors may be a crucial component of surfactant homeostasis and maintenance of lung function in both premature and mature lungs (Hokuto and others 2004).

FGF Signaling and the Upper Airway

Most airway anomalies arise from early lung development defects resulting in abnormal fistulous tracts and cartilaginous malformations. The cartilaginous support structures of the upper airway include the larynx and trachea and help maintain upper airway patency (Elluru and others 2009). Congenital upper airway malformations occur during the formation of the fetal foregut during the first weeks of embryonic development. Subsequently, fistulous tracts derived from the respiratory epithelium extend during the caudal growth phase up to ∼16 weeks of gestational age. FGF 7 and FGF2RIIIb receptor defects have been implicated in animal models. In one study of tracheoesophageal fistula formation in rats, levels of FGF2R mRNA levels were lower and similar to esophageal levels, rather than those of normal control lungs (Spilde and others 2004). In a study using overexpression of FGF 18 in chondrocytes, the authors showed that FGF 18 provides directional and proliferative signals via upregulation of the transcription factor pathway. FGF 18, FGF 7, and FGF2R may be novel targets in studying the development of the cartilaginous tissue forming the upper airway (Elluru and others 2009).

FGF Signaling and Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a complex disease with many contributing factors, resulting in a cellular and structural level in inflammation, remodeling, and alveolar space enlargement. Altered transforming growth factor-β (TGF-β) signaling has been implicated in the development and worsening of emphysema. Environmental factors such as smoke exposure may reduce SMAD3 and SMAD7 expression in cultured fibroblasts. Based on animal studies, the loss of integrin activation of TGF-β leads to production of matrix metalloproteinases and emphysema in mice (Morris and others 2003; Shi and others 2006). FGF has been shown to alter TGF-β regulated fibrosis and, thus, may be an important therapeutic target in COPD. Further studies are needed in this area to identify FGF matrix therapeutic targets. There is recent evidence that common signaling pathways and gene expression exist between asthma and COPD. TGF-β1 has been found to be one growth factor that is upregulated in both asthma and COPD. It is functionally associated with airway wall thickening as demonstrated by computed tomography scan (Postma and others 2011). The consideration of TGF-β1 as a potential therapeutic target for treatment of airway remodeling refractory to corticosteroids is currently under investigation. Monoclonal antibodies to TGF-β1 that block pSMAD2 signaling are being investigated to determine whether airway smooth muscle mass, collagen deposition in the lung are altered (McMillan and others 2005). Similarly, the regulation of FGF expression may be a therapeutic approach to prevent aberrant airway collagen deposition.

FGF and Viral Induction of Remodeling

Viral infection of airway epithelial cells may result in increased production of growth factors and collagen deposition, thickened basement membrane, and other cellular findings of airway remodeling.

FGF and the FGF signaling pathway may serve as a counterbalance to pro-fibrotic signals such as TGF-β1. The transition from epithelium to mesenchymal tissue (EMT) is largely under the control of growth factors. Certain growth factors inhibit the transition and prevent accumulation of the myofibroblast phenotype and collagen in the airway. Hepatocyte growth factor has been shown to upregulate SMAD7 and potentially revert the EMT process (McMillan and others 2005; Ramos and others 2010; Postma and others 2011). Similarly, in a study of FGF 1, it was shown that FGF 1 is an antagonist of the TGF-β1 regulated EMT process. Specifically, FGF 1 inhibits SMAD2 phosphorylation through the protein kinase signaling pathway.

After adenoviral and other severe viral infections, obliteration of bronchioles may occur, thus resulting in clinical irreversible airways disease and obliteration of small noncartilaginous airways. In an immunohistochemical study of bronchiolitis obliterans organizing pneumonia, FGF 2 was widely expressed and co-expressed with VEGF. The presence of these growth factors in fibromyxoid connective tissue and its association with angiogenesis suggest a role in pathogenesis (Shukla and others 2009). In addition to angiogenesis and formation of granulation tissue, the FGF family regulates fibroblast proliferation and production of collagen (McGee and others 1988; Pierce and others 1992; Lappi-Blanco and others 2002).

Viral illness can result in infection of airway cells and contribute to the exacerbation of lung diseases such as COPD and asthma. There is also some evidence that certain viruses such as respiratory syncytial virus (RSV) can contribute to the development of prolonged wheezing in susceptible hosts (Martinez 2000).

In some asthmatic patients, there is evidence that remodeling and EMT can even occur early in the disease process and in young patients (Weckmann and others 2009). In our previous study of RSV, epithelial cells were infected with the virus and inactivated virus. The cells exposed to either replicating virus or inactivated viral elements produced significantly higher levels of FGF-β (Dosanjh and others 2003). These findings suggest that FGF-β may be an important response to viral injury of epithelium during illness. The FGF signaling pathway may provide potential therapeutic targets in counterbalancing the aberrant TGF-β1 pro-fibrotic response to viral infection.

Asthmatic Airway Remodeling and FGF 2

Hypertrophy of the airway smooth muscle (ASM) is a hallmark pathologic finding in asthmatic remodeling. The cellular signals required for ASM proliferation involve synergy between TGF-β and FGF 2. Cellular studies suggest that FGF 2 is overexpressed in the lung and detectable shortly after allergen exposure. In contrast, TGF-β lung lavage concentration peaks 24 h after segmental allergen exposure (Redington and others 1997). A synergistic mechanism of TGF activation by FGF 2 has been suggested by cellular studies which demonstrate that in the absence of FGF 2, TGF-β is less potent in effecting ASM proliferation (Cohen and others 2000). This and other studies have shown that TGF-β requires the presence of growth factors to induce ASM proliferation. The presence of TGF-β alone is not sufficient to cause ASM cell hyperplasia, as it requires activation by identified factors that induce post-translational modifications to mediate its end effect (Bosse and others 2010). Although there is a study by Chen and Khalil (2006) demonstrating that neutralizing antibodies to FGF 2 had no effect on TGF-β induced ASM mitogenesis, there was another study by Black and others (1996) in which the pulse timing of thymidine was delayed by 12 h, showing TGF-β became antimitogenic in its cellular function (Black and others 1996; Chen and Khalil 2006). Considered together, this may suggest that the role of FGF 2 may be to reverse the antimitogenic delayed effects of TGF-β after activation. These studies suggest that FGF 2 signaling is important in regulation of ASM cellular hyperplasia, a key feature of refractory asthma, and should be further studied as a therapeutic target.

Conclusions

A number of cellular, molecular, animal, and human studies suggest the importance of considering FGF signaling in the development of normal lung and in a variety of obstructive and interstitial lung diseases. New therapies of targeted FGF candidate genes may hold promise in treating infant, pediatric, and adult airway disease.