OR WAIT null SECS
Abnormal Iron Homeostasis in Alveolar Macrophages: Implications for COPD Pathogenesis
Chronic obstructive pulmonary disease (COPD) is the third leading cause of death from non-communicable diseases worldwide1. Although cigarette smoke (CS) exposure is known to trigger COPD development, many other biological aspects of COPD pathogenesis are unknown, preventing the development of personalized therapies that target specific mechanistic endotypes.
We had previously shown that there are elevated levels of iron and iron proteins in the airways of smokers and subjects with COPD which associate with clinical COPD parameters including exacerbation frequency2. The source of these iron biomarkers is likely airspace macrophages (AMs), with various studies showing that AMs from smokers and COPD patients are overloaded with iron and can release iron ex vivo3–5. We hypothesized that this AM abnormal iron metabolism may be a critical driver of COPD pathobiology, and specific to this proposal, that AM iron accumulation contributes to the well-known phenomenon of AM dysfunction in COPD6, resulting in an increased risk for infection and infection-triggered COPD exacerbation.
We replicated CS-induced AM iron loading using a murine model of CS exposure and showed that at the transcriptomic and proteomic levels that there is dysregulation of critical iron metabolic proteins. We then utilized single cell RNA-sequencing and identified unique subpopulations of macrophages which have this iron dysregulated signature. Interestingly, this iron signature is accompanied by the differential expression of critical genes related to AM identity and immune function, suggesting that iron dysregulation have consequences for these cells beyond iron metabolism alone. This supports our hypothesis that disruption of AM iron homeostasis from smoke may potentiate respiratory infections.
To test this hypothesis, we developed a transgenic mouse model that is depleted of an important iron regulatory protein, nuclear coactivator 4 (NCOA4). NCOA4 has indispensable roles in sensing cellular iron levels and degrading the iron storage protein ferritin to retrieve iron in times of high iron requirements7. AM NCOA4 depletion abates CS-induced AM iron accumulation, and we are in the process of completing our image analysis of inflated lung sections from long-term (6-8 months) CS-exposed AM NCOA4-depleted and control mice to determine whether NCOA4 depletion alone is sufficient to deter emphysema development.
One of the major differences between murine COPD models and human COPD is that COPD in patients progresses in a stepwise fashion in the form of exacerbations. To mimic this, we exposed mice to short (8-12 weeks) periods of smoke and then instilled them with Streptococcus pneumoniae as a second hit to model infection-associated exacerbation. Although mice with AM NCOA4 depletion seemed initially to have improved survival from S. pneumoniae infection, with more replicate experiments it does not seem that they are protected. This highlights the critical importance of maintaining iron homeostasis as while CS-induced iron overload may have adverse effects on macrophage function, the inability to access ferritin-stored iron during an acute infection may limit AMs energetically and metabolically, preventing efficient bacterial clearance8.
This work has been presented at the American Thoracic Society (ATS) International Conference. We have completed all experiments in the murine component of this project and are currently at the manuscript preparation stage and expect to submit this work for review later this year. Consistent with the focus of our laboratory, during the funding period of this project we published 2 additional manuscripts that centers on the effect of CS on the AM inflammatory response9,10.
We are in the process of translating some of our murine findings back to the bedside. We showed in a proof-of-concept study that we could use deferiprone, an iron chelator, to reduce intracellular iron levels in AMs obtained from bronchoalveolar lavage in smokers but not in subjects with COPD11. Whether this difference is due to fundamental biological differences or due to inadequate dosing and bioavailability is the subject of ongoing studies.
We are also finalizing our manuscript that evaluates AMs transcriptomically in smokers and subjects in COPD in the multicenter Subpopulations and Intermediate Outcome Measures in COPD Study (SPIROMICS) cohort, with interesting findings replicating the findings in our murine transcriptomic studies. This work has already been presented at ATS and should be ready for submission in the next few months.
Zhang has no relevant disclosures to report.
