Emerging roles of ferroptosis in pulmonary fibrosis: current perspectives, opportunities and challenges

Abstract

Pulmonary fibrosis (PF) is a chronic interstitial lung disorder characterized by abnormal myofibroblast activation, accumulation of extracellular matrix (ECM), and thickening of fibrotic alveolar walls, resulting in deteriorated lung function. PF is initiated by dysregulated wound healing processes triggered by factors such as excessive inflammation, oxidative stress, and coronavirus disease (COVID-19). Despite advancements in understanding the disease's pathogenesis, effective preventive and therapeutic interventions are currently lacking. Ferroptosis, an iron-dependent regulated cell death (RCD) mechanism involving lipid peroxidation and glutathione (GSH) depletion, exhibits unique features distinct from other RCD forms (e.g., apoptosis, necrosis, and pyroptosis). Imbalance between reactive oxygen species (ROS) production and detoxification leads to ferroptosis, causing cellular dysfunction through lipid peroxidation, protein modifications, and DNA damage. Emerging evidence points to the crucial role of ferroptosis in PF progression, driving macrophage polarization, fibroblast proliferation, and ECM deposition, ultimately contributing to alveolar cell death and lung tissue scarring. This review provides a comprehensive overview of the latest findings on the involvement and signaling mechanisms of ferroptosis in PF pathogenesis, emphasizing potential novel anti-fibrotic therapeutic approaches targeting ferroptosis for PF management.

Fig. 1. Risk factors of PF.

Numerous risk factors have been implicated in the development of PF, including exposure to tobacco smoke, gastroesophageal reflux, viral and bacterial infections (such as COVID-19), toxic substances (such as asbestos, silicon dioxide, PM2.5, and PQ), genetic variations, and immune disorders.

Fig. 2. Molecular insights into PF pathophysiology.

Repeated injury to AECs leads to chronic inflammation, which is considered to be the initiating event of PF, followed by ACE-II EMT, neutrophil infiltration, and macrophage polarization. A multitude of pro-fibrotic mediators (such as TGF-β, IL-1β, TNF-α, and PDGF) are then released, leading to fibroblasts proliferation and the EMT of AECs. These molecular events progressively triggering macrophage polarization, fibroblast proliferation, and myofibroblast activation. Subsequently, the over-synthesis of ECM components by myofibroblasts contributed to reduced lung compliance and ultimately irreversible PF.

Fig. 3. Molecular mechanisms of ferroptosis.

TfR1 mediates the endocytosis of Tf-Fe³⁺ into lysosomes for iron uptake. Subsequently, Fe³⁺ is converted to Fe²⁺ by STEAP3 and transported into the labile iron pool via DMT1. Additionally, Fe²⁺ can be sequestered by ferritin after being converted to Fe³⁺ through PCBP-mediated processes. In cases of intracellular iron deficiency, NOCA4-mediated ferritinophagy restores the levels of available iron ions. However, when free ferritin ions enter mitochondria via DMT1, they induce oxidative stress. The Fenton reaction, facilitated by iron, generates substantial amounts of reactive oxygen species (ROS), leading to lipid peroxidation primarily targeting polyunsaturated fatty acids (PUFAs). PUFA peroxidation necessitates the involvement of ACSL4 and LPCAT3 enzymes and ultimately triggers ferroptosis. Cyst(e)ine/GSH/GPX4 axis regulates ferroptosis by mitigating the detrimental effects of lipid peroxidation through its reduction back to lipids. Impaired GPX4 function or inhibition of system xc- activity along with depleted GSH levels result in lipid peroxide accumulation and subsequent ferroptotic cell death.

Fig. 4. Iron homeostasis in the lungs.

The iron present in foods primarily exists as heme-Fe²⁺ and nonheme iron (Fe³⁺). Fe³⁺ is reduced by Dcytb in the brush-border membrane and subsequently transported into enterocytes via DMT1. Heme-Fe²⁺ is absorbed and degraded within enterocytes by HO-1. Once exported through FPN, Fe²⁺ undergoes rapid conversion to Fe³⁺ by HEPH and binds to transferrin for circulation. The majority of Fe³⁺ is bound to transferrin, which is taken up by TfR1 at the surface of AECs, followed by reduction of Fe³⁺ to Fe²⁺ by STEAP3 and export into the labile iron pool (LIP) in the cytosol via DMTI. ZIP8 predominantly localizes at the apical surface of AECs, facilitating transport of non-transferrin-bound iron from the airways into AECs. Ferritin serves as a large iron storage site regulated by PCBP. Under conditions of iron deficiency, NCOA4 mediates ferritinophagy to increase intracellular iron levels.

Fig. 5. Endogenous ferroptosis inhibitory systems.

Lipid peroxidation of membrane phospholipids can be eliminated by several endogenous ferroptosis inhibitory systems, such as Cyst(e)ine/GSH/GPX4 system, NADPH/FSP1/CoQ10 system, GCH1/BH4/DHFR system, and GPX4/DHODH system.