Role of iron in the pathogenesis of lung disease

Ali, Md Khadem

Title: Role of iron in the pathogenesis of lung disease
Creator: Ali, Md Khadem
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2018
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: Iron is a key bio-metal required for almost all living organisms. It plays a role in many different biological processes. Increasing evidence suggests that iron dyshomeostasis is linked with a range of lung diseases, such as idiopathic pulmonary fibrosis (IPF), asthma, cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD) and lung cancer. However, the mechanisms that underpin these associations and the role that iron plays in the pathogenesis of these lung diseases are yet to be fully elucidated. In Chapter 1 of my thesis, I outline the current literature highlighting an association between altered iron and respiratory disease. This introductory chapter lays the foundation for the body of work conducted during my PhD candidature, which explored the role of iron in lung disease using a combination of murine models of altered iron loading and pulmonary fibrosis and asthma and the complementary analyses of some clinical samples. Using both genetically-(transferrin receptor [TFR] 2 mutant [TFR2^mut/mut] and haemochromatosis protein [HFE]-knockout [HFE^-/-] mice) and dietary-(wild-type [WT] BALB/c mice fed a 2% carbonyl iron diet)-induced systemic iron overloading models, in Chapter 2 of my thesis, I extend upon findings from the literature to show that systemic iron overloading leads to an increase in iron accumulation in the lungs that is associated with alterations in iron regulatory molecule expression. Importantly, I show that increased iron levels in the lung are associated with emphysema-like alveolar enlargement, small airways collagen deposition, inflammation, impairment of cilia beat frequency and increased bacterial infection, alterations in baseline lung function and increased airways hyper-responsiveness (AHR). These novel observations are important as the changes in lung structure and/or function that I have observed with increased iron levels in the lung are hallmark features of a number of respiratory diseases including IPF, asthma and COPD. Interestingly, I show no change in iron levels in the lung of WT BALB/c mice fed low iron diet but increased HAMP and TFR1 mRNA expression despite these mice having decreased systemic iron levels. However, mice fed a low iron diet have lower levels of collagen deposition around the small airways and AHR compared to mice fed a normal diet. Together, my findings demonstrate an important role for iron homeostasis, both systemically and in the lung, in the pathogenesis of different features of lung disease. Given the effects of iron levels on airway fibrosis, I next sought to explore the relationship between pulmonary iron levels and IPF using a well-established bleomycin-induced murine model of pulmonary fibrosis and TFR2^mut/mut mice. Interestingly, I show that, whilst the bleomycin-induced pulmonary fibrosis model has no effect on systemic iron levels in WT AKR mice, there is a significant increase in lung iron associated with disease, and the levels observed in the lungs of bleomycin-treated WT AKR mice is similar to that observed in TFR2^mut/mut mice. Significantly, I show that TFR2^mut/mut mice that have not been treated with bleomycin have similar levels of pulmonary fibrosis in the small airways and AHR as bleomycin-treated WT AKR controls. This suggests that increased iron levels in the lungs are sufficient for inducing some of the hallmark features of a commonly utilised pulmonary fibrosis model. Importantly, I show that intranasal treatment with the iron chelator, deferoxamine (DFO), ameliorates airway inflammation, small airway fibrosis, decreased carbon monoxide (CO) diffusion capacity, and AHR in WT BALB/c mice treated with bleomycin, suggesting the treatments that target iron accumulation in the lungs could prove promising therapeutic strategies for diseases associated with pulmonary fibrosis such as IPF. In Chapter 3, I assessed broncho-alveolar lavage fluid (BALF) supernatant, BALF cells and airway tissues collected from patients with asthma and healthy controls to demonstrate for the first time that iron levels in the BALF supernatant is significantly and positively associated with FEV1% predicted (r=0.4367, [p=0.0257]) of asthmatics and healthy subjects. Interestingly, iron positive BALF cells negatively correlate with FEV1/FVC% (r=-0.4635, [p= 0.0298]). Together, these findings suggest that iron homeostasis in the airways is linked with airflow obstruction. I also show that mRNA expression of DMT1 IRE and TFR1 mRNA expression in airways tissues (r=-0.6149, [p= 0.0039] and r=-0.5727, [p=0.0083], respectively) are strongly and negatively associated with FEV1/FVC%. Using a house dust mite (HDM)-induced murine model of experimental asthma, I demonstrate that experimental asthma results in increased iron levels in the lung. Together, these clinical and experimental findings indicate that increased iron levels in lung tissues may be a consequence or driver of allergic asthma and may be associated with the severity of disease. I also demonstrate that HFE^-/- mice, which also have increased iron levels in the lungs, have increased airway inflammation (total cells in BALF), IL-13 mRNA expression, small airway fibrosis, and AHR during HDM-induced experimental asthma compared to HDM-treated, WT AKR controls. Significantly, I also show that mice fed a high iron diet have significantly increased HDM-induced AHR, small airways fibrosis, and tissue eosinophilic inflammation compared to mice fed a normal diet. This is despite the high iron diet, not increasing lung iron levels during HDM-induced experimental asthma compared to mice with experimental asthma fed a normal diet. Interestingly, whilst, a low iron diet did not have any effects on lung iron levels in control mice without experimental asthma compared to control mice fed a normal diet, mice fed a low iron diet had decreased lung iron during HDM-induced experimental asthma. This was associated with reduced inflammatory and mucus secreting cell numbers in the airways lumen but not airways fibrosis or AHR. Using an ovalbumin (Ova)-induced model of experimental asthma, I also demonstrate that mice fed a high iron diet have worsened Ova-induced experimental asthma, including increased IL-5 mRNA expression in the lung, AHR, tissue eosinophilic inflammation, mucus secreting cells metaplasia. Together, these findings suggest that increasing iron levels both systemically and/or in the lung, through both high iron diet and genetic deletion of HFE, results in more severe disease in two different models of experimental asthma. Significantly, I show that high iron diet-induced severe Ova-induced experimental asthma is resistant to steroid (dexamethasone) treatment and that treatment with DFO suppresses key features of experimental asthma in both HDM- and Ova-induced models of experimental asthma. Collectively, these clinical and experimental findings suggest that iron homeostasis plays an important role in the pathogenesis of many of the key features of asthma with increased iron levels both systemically and in the lung tissue/cells being an important driver and/or consequence of disease. These findings also show that increased iron levels in lung tissue/cells may result more severe features of disease that are resistant to steroid therapy and that targeting/correcting altered iron homeostasis in asthma may be a therapeutic option for the treatment of disease.
Subject: iron; asthma; small airway fibrosis; lung fibrosis; airways hyper-responsiveness; respiratory disease; iron chelator; inflammation; thesis by publication
Identifier: http://hdl.handle.net/1959.13/1391443
Identifier: uon:33233
Language: eng
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