We investigated the potential role of FGF2 in the development of IFN-γ-induced emphysema using two emphysema models: a lung-targeted IFN-γ TG model and a severe non-eosinophilic asthma model induced by airway exposure to LPS-containing allergens. Cross-breeding with IFN-γ TG and FGF2 KO mice showed that IFN-γ-induced emphysema was aggravated by the absence of FGF2. In addition, treatment with rFGF2 prevented the development of emphysema in the severe asthma model, which depends in part on IFN-γ.
Asthma can be divided into two inflammatory subtypes: eosinophilic and non-eosinophilic (Wenzel et al., 1999
; Lemiere et al., 2006
) and the most consistent cellular finding in severe asthmatics is the high number and percentage of neutrophils in induced sputum, BAL, and biopsy specimens (Wenzel et al., 1997
; Elias, 2004
; Kim et al., 2007b
). Our previous clinical studies showed that the expression of IFN-γ, but not IL-4, was enhanced in severe asthma patients compared to patients with mild and moderate asthma (Kim et al., 2007b
). In addition, asthma animal models using allergen inhalation and lung-targeted IFN-γ transgenes have demonstrated that IFN-γ is a key mediator in the development of non-eosinophilic asthma phenotypes (Kim et al., 2007a
). Furthermore, the present data showed that non-eosinophilic inflammation induced by LPS-containing allergens depended on IFN-γ. Taken together, these findings suggest that IFN-γ is a key mediator in the development of severe non-eosinophilic asthma.
While on average and in the extremes of presentation, asthma and chronic obstructive pulmonary disease (COPD) appear to be easily distinguishable, significant proportions of patients are indistinguishable on the basis of structural and functional evaluations (Sciurba, 2004
). Asthma patients with persistent maximum expiratory airflow limitations, despite optimal poly-therapy, have a marked loss of lung elastic recoil, which is related to emphysema in COPD (Gelb et al., 2008
). In terms of alveolar remodeling, asthma is generally considered a disease with normal alveoli; in contrast, alveolar destruction with secondary alveolar enlargement is a characteristic feature of emphysema (Elias, 2004
). Studies of emphysematous human tissues have highlighted alterations that heighten tissue proteolysis, such as increased elastin degradation and enhanced expression of proteases (Ohnishi et al., 1998
; Takeyabu et al., 1998
). However, these changes have also been documented in severe asthma patients, leading to COPD-like alterations in lung compliance (Bousquet et al., 1996
; Gelb and Zamel, 2000
). These findings suggest that alveolar remodeling (such as that which is associated with emphysema) in severe asthma patients can lead to fixed or irreversible airway obstruction.
When IL-13 is over-expressed in the murine lung, both asthma phenotypes (e.g., eosinophilic inflammation, mucous metaplasia, and AHR) and COPD phenotypes (e.g., bronchiolar fibrosis and alveolar enlargement) are obvious (Zhu et al., 1999
). In contrast, the TG expression of IFN-γ in the murine lung causes neutrophilic inflammation and impressive pulmonary emphysema without fibrosis (Wang et al., 2000
). In the present study, repeated airway exposure to LPS-containing allergens induced non-eosinophilic inflammation and alveolar remodeling, as in emphysema. Furthermore, these phenotypes induced by LPS-containing allergens depended on IFN-γ but not IL-13. These findings suggest that IFN-γ is a key mediator in the development of the fixed airway obstruction seen in patients with severe non-eosinophilic asthma.
Cigarette smoke-induced emphysema animal models were used to elucidate the pathogenesis of emphysema and to discover therapeutic candidate drugs because of the strong correlation between cigarette smoking and emphysema (Turner and Grose, 2010
). However, not all emphysemas can be explained by the cigarette smoke emphysema model (Joos et al., 2002
). The emphysema seen in the LPS-containing allergen-induced severe asthma model in the present study may be an alternative model to back up the shortcomings of the cigarette smoking models. The so-called Dutch hypothesis suggests that asthma and COPD are not distinct entities, and that similar pathogenic mechanisms may be involved in the pathogenesis of asthma and COPD in some individuals (Sluiter et al., 1991
; Elias, 2004
). The present severe asthma model supports this idea in that the progression of severe asthma leads to emphysema or COPD.
IFN-γ in the lungs induces pulmonary destruction through the production of various proteases (Zheng et al., 2005
) and apoptosis of lung epithelial cells (Lesur et al., 2004
). The chronic inflammatory process causes tissue injury followed by healing (Lee et al., 2002
). It is generally accepted that the modulation of fibroblastic cells toward the myofibroblastic phenotype, in combination with the acquisition of specialized contractile features and the enhanced production of extracellular matrix proteins, is essential for connective tissue remodeling during wound healing (Holgate et al., 2000
). TGFβ1 is a key mediator in wound healing and in the fibroblast-to-myofibroblast phenotypic change, (Morishima et al., 2001
) and a key antagonist of IFN-γ (Lin et al., 2005
). High levels of TGFβ1 in the lungs cause prominent pulmonary fibrosis rather than destruction (Lee et al., 2006
). These data suggest that IFN-γ-induced emphysema can be inhibited by TGFβ1.
FGF2 is a downstream mediator of TGFβ1 (Jeon et al., 2007a
). The present data also showed that TGFβ1 induced in vitro production of FGF2 from fibroblasts. During lung branching morphogenesis (or lung modeling) or remodeling, the local production of cytokines and growth factors by epithelial and mesenchymal cells effectively creates an epithelial-mesenchymal trophic unit (Holgate et al., 2000
). Interactions between the FGF family and their receptors can mediate mesenchymal-epithelial cell interactions during lung modeling and remodeling (Sekine et al., 1999
). FGF2 plays a role in wound healing by stimulating fibroblast differentiation and the regeneration of epithelial cells from endogenous or exogenous stem/progenitor cells (Warburton et al., 2008
). Because lung inflammation and emphysema induced by transgenic IFN-γ in the lung was aggravated by the absence of FGF2, we hypothesized that rFGF2 treatment inhibits severe asthma phenotypes, such as lung inflammation and emphysema induced by LPS-containing allergens. The present data showed that rFGF2 treatment significantly inhibited the development of emphysema but not lung inflammation. Taking into consideration the induction of apoptosis in lung epithelial cells by IFN-γ, these data suggest that the protective effects of FGF2 against the development of emphysema are related to the enhanced regeneration of epithelial cells.
In summary, the present study suggests that IFN-γ is a key mediator in the development of emphysema seen in severe non-eosinophilic asthma, and FGF2 helps to protect against the development of IFN-γ-induced emphysema. In terms of clinical application, recombinant FGF2 may be a promising therapeutic agent for the treatment of emphysema progression seen in severe asthma and COPD.