Respiratory Pathophysiology Through the Lens of Mitochondria

Abstract

Mitochondrial integrity is indispensable for pulmonary cellular homeostasis, with its dysfunction increasingly being implicated as a central mechanism in the etiology of respiratory disorders. We present a comprehensive overview of the integral role played by mitochondrial dynamics, such as fusion, fission, mitophagy, intracellular trafficking, and biogenesis, in maintaining pulmonary homeostasis. This study further explores how perturbations in these processes contribute to the pathogenesis of diverse lung disorders, including chronic obstructive pulmonary disease (COPD), bronchopulmonary dysplasia (BPD), pulmonary arterial hypertension (PAH), idiopathic pulmonary fibrosis (IPF), and drug-induced lung disease. It further explores how perturbations in these processes contribute to the pathogenesis of diverse lung disorders—for example, chronic obstructive pulmonary disease (COPD; responsible for roughly 55% of chronic respiratory disease cases), bronchopulmonary dysplasia (BPD; affecting up to 45% of infants born before 29 weeks of gestation), pulmonary arterial hypertension (PAH; a rare condition causing about 22,000 deaths worldwide in 2021), idiopathic pulmonary fibrosis (IPF; 0.33–4.51 cases per 10,000 persons), and drug-induced lung disease. Evidence demonstrates that mitochondria-triggered apoptosis, metabolic shifts, and subsequent inflammatory signaling act together to drive airway tissue remodeling and fibrotic progression across these lung diseases. Furthermore, this review evaluates the therapeutic potential of mitochondrial-targeted drugs, such as MitoQ and SS31, and metformin, which have shown promise in basic and preclinical studies. Preclinical and early clinical evaluations include an ongoing trial of the mitochondrial-targeted antioxidant MitoQ (NCT02966665, phase 1) in COPD, a 4-month open-label DCA study in PAH patients, and studies determining the preclinical efficacy of SS-31 and metformin in IPF models. Ultimately, integrating mitochondrial biomarkers into clinical practice holds the potential not only to facilitate early disease detection but also to enable the development of precision therapies, thereby offering renewed hope for patients afflicted with chronic lung diseases.

1. Introduction

Lung disorders continue to rank among the leading causes of death globally, remaining a significant and ongoing public health concern. Based on data from GBD Study 2019 (global burden of chronic respiratory diseases and risk factors), approximately 454.6 million people (95% UI: 417.4–499.1 million) suffered from chronic respiratory diseases in 2019, and about 4.0 million deaths (95% UI: 3.6–4.3 million) were attributed to these conditions, corresponding to an age-standardized mortality rate of around 51.3 deaths per 100,000 people [1]. Although the crude numbers have increased due to population growth since 1990, the age-standardized rates have generally declined, suggesting overall improvements in relative health outcomes. Despite general improvements in some areas, specific lung disorders continue to pose a challenge; therefore, the persistent global burden of lung-related disorders underscores the critical need for innovative biomarkers and therapeutic approaches to further enhance patient outcomes.

Mitochondria represent a particularly promising target for therapeutic intervention aimed at modulating cell fate. In addition to generating ATP via oxidative phosphorylation (OXPHOS), they regulate metabolism, mediate calcium and redox signaling, and control apoptosis—all processes that influence cell fate [2,3,4]. Furthermore, recent studies have shown that mitochondrial cytochrome c oxidase subunit 4 isoform 2 is essential for hypoxia-induced ROS release and acute oxygen sensing in both the carotid body and the pulmonary vasculature [5]. Given the multifaceted role played by mitochondria, revealing mitochondria-associated pathways dysregulated in lung disease could provide useful insights for disease detection and monitoring, as well as developing targeted therapeutic strategies.

Mitochondrial dysfunction is involved in various pulmonary conditions, such as chronic obstructive pulmonary disease (COPD), IPF, and pulmonary arterial hypertension (PAH), and targeting mitochondria with specific drugs (such as mitochondrial-targeted antioxidants MitoQ) may help to reduce oxidative stress and inflammation in these disorders (

Table 1. An overview of therapeutic mitochondrial drugs used for treating lung disease in this review.

Drug/Approach	Mechanism of Action	Target Disease	Citation, Notes
BGP-15	Activator of OPA1	COPD	[6]
Leflunomide	DHODH inhibitor	COPD	[7]
MitoQ	Mitochondria-targeted antioxidants	COPD, PAH	[8,9]
Dichloroacetate	PDK inhibitor	PAH	[10]
Rosiglitazone	PPARγ agonist	IPF	[11]
SS-31	Cell-permeable mitochondrial-targeted peptide	IPF	[12,13]
ABT-199	Antiapoptotic factor Bcl-2 inhibition	IPF	[14]
Metformin	Mild inhibition of respiratory chain complex I	IPF	[15,16,17,18]
Coenzyme Q10 α-Tocopherol	Mitochondria-targeted antioxidants	Amiodarone- induced toxicity	[19]
Mito-MEN	Mitochondria transfer system	Bleomycin-induced pulmonary fibrosis	[20]

). However, while preclinical studies are promising, definitive clinical evidence is still lacking, and further research is needed to establish their safety and efficacy for practical therapeutic use in lung diseases. Beyond their pathological implications, the physiological roles of mitochondria in the lungs are increasingly gaining recognition. This review synthesizes key insights from the existing literature and highlights emerging paradigms in mitochondrial biology and pathology within the pulmonary system.

2. Mitochondrial Dynamics and Relevant Factors Involved in Lung Pathophysiology

Mitochondria are dynamic organelles that autonomously engage in cycles of fission and fusion, thereby sustaining cellular homeostasis, known as mitochondrial dynamics [21] (Figure 1). Recent advances in pathobiology have illuminated the central role played by these dynamic processes in orchestrating a diverse array of cellular responses, ranging from programmed cell death to differentiation. Mitochondrial dynamics profoundly influence cellular fate and overall organismal viability by modulating key metabolic pathways. In particular, by shaping mitochondrial morphology and cristae architecture, these dynamics optimize OXPHOS. Embedded in the inner mitochondrial membrane, five respiratory chain complexes (RCCs; complexes I–V) drive ATP synthesis. Complex I forms a supercomplex with complexes III and IV to organize electron flux and optimize the use of available substrates [22]. The RCCs’ proton-pumping-coupled electron transfer reactions are summarized below [23]:

Complex I: NADH + H⁺ + Q + 4H⁺ (matrix) → NAD⁺ + QH₂ + 4H⁺ (inner membrane space; IMS);

Complex III: 2Cyt.c (oxidized) + QH₂ + 2H⁺ (matrix)→ 2Cyt.c (reduced) +Q+4H⁺ (IMS);

Complex IV: 4Cyt.c (reduced) + 8H⁺ (matrix) + O₂ → 4Cyt.c (oxidized) +2H₂O + 4H⁺ (IMS).

These protons, which are transported out of the mitochondrial matrix, generate the electrochemical proton gradient across the inner membrane to synthesize ATP via the ATP synthase of complex V [24].

Dynamic mitochondrial networks are orchestrated by a family of GTP-dependent dynamin-related proteins, which govern their structural remodeling and functional integrity [25]. DRP1 (dynamin-related protein) is the main regulator of mitochondrial division. Its dimers and oligomers form spiral-like assemblies around the prospective fission site. Upon GTP hydrolysis, the helical pitches and diameters of these DRP1 oligomers decrease, thereby constricting and ultimately splitting the mitochondrion [26,27].

Mitochondria maintain cellular homeostasis through balanced cycles of fission and fusion, regulated primarily by DRP1 (fission) and MFN1/2 with OPA1 (fusion). OPA1, together with PHB and MICOS (mitochondrial contact site and cristae organizing system) complexes, also shape cristae to support efficient oxidative phosphorylation. Mitochondrial transport relies on MIRO1, which coordinates microtubule-based trafficking to meet local energy demands. Quality control is sustained by factors such as PINK1 and Parkin, facilitating mitophagy, and LONP1 (lon peptidase 1, mitochondrial), which contributes to proteolytic maintenance. Meanwhile, PGC-1α drives mitochondrial biogenesis by inducing transcription factors that expand and renew the mitochondrial population. The interplay among these processes is critical for preserving mitochondrial integrity and function, ultimately influencing lung pathophysiology through effects on cell survival, metabolism, and stress responses. Diseases linked to each mitochondrial factor are shown within red-outlined boxes.

Outer mitochondrial membrane (OMM) fusion is driven by MFN (mitofusin)1 and MFN2. These proteins assemble into both homo- and heterodimeric complexes. They undergo GTP hydrolysis-dependent conformational changes that bring the membranes together and enable their fusion [28,29]. OPA1 (Optic Atrophy 1) orchestrates the fusion of the inner mitochondrial membrane (IMM) [30,31] and sculpts the cristae where respiratory chain complexes are housed [32,33]. The OPA1-dependent cristae architecture is crucial for assembling respiratory chain supercomplexes and for maintaining stable and efficient mitochondrial respiration [34]. The earliest mitochondrial SPFH (stomatin, prohibitin, flotillin, HflC/K) proteins identified were prohibitins, consisting of alternating PHB1 and PHB2 subunits [35]. By stabilizing OPA1, prohibitin complexes also support mitochondrial translation and the remodeling of cardiolipin (CL). Prohibitin maintains the mitochondrial architecture and ensures efficient respiratory function to confer resistance to apoptosis [36]. In addition to OPA1, other membrane-shaping factors—the dimeric F1Fo-ATP synthase and the mitochondrial contact site and cristae organizing system (MICOS)—contribute critically and distinctly to the biogenesis and maintenance of cristae [37,38]. OPA1 is epistatic to MIC60 (MICOS component 60), controlling the crista junction number, stability, and width to unify mitochondrial cristae biogenesis and remodeling [39]. Cells finely tune mitochondrial movement to ensure their proper distribution according to local energy demands. Rhot1 (ras homolog family member 1), also called MIRO1 (Mitochondrial Rho GTPase 1), encodes an atypical Ras GTPas, which is central to this process, regulating intracellular mitochondrial trafficking via Ca²⁺ signaling. Under normal conditions, MIRO1 associates with Milton and Kinesin-1 (KHC) to form a complex that efficiently transports mitochondria along microtubules [40].

Maintaining optimal mitochondrial function is essential for cellular health, and this is achieved through mitophagy—the selective degradation of mitochondria via autophagy. Mitophagy is initiated either via specific receptors on the OMM or through the attachment of ubiquitin molecules to mitochondrial proteins, facilitating the formation of autophagosomes that encapsulate the organelles. Mitophagy operates through two primary pathways: the receptor-mediated and ubiquitin-dependent pathways. Among the receptors involved in receptor-mediated mitophagy, BNIP3 (Bcl-2 interacting protein 3) and NIX serve as the principal regulators, while FUNDC1 (FUN14 Domain Containing 1), BCL2L13 (BCL2-like 13), and FKBP8 (FK506-binding protein 8) play less dominant roles [41]. The receptors interact with LC3 (microtubule-associated protein 1A/1B-light chain 3)/GABARAP proteins to target mitochondria for degradation. Furthermore, the PINK1 (PTEN-induced kinase 1)–Parkin pathway labels mitochondrial proteins with ubiquitin, while regulatory kinases, such as CK2 (casein kinase 2) and ULK1 (Unc-51-like autophagy activating kinase 1), along with phosphatases, such as PGAM5 (phosphoglycerate mutase family member 5), adjust receptor activity to ensure that only damaged mitochondria are selectively removed [42].

Mitochondrial biogenesis—a process that increases both the number and size of mitochondria—is primarily orchestrated by the transcriptional coactivator PGC-1α (PPAR-γ coactivator-1α). This key regulator activates the nuclear respiratory factors NRF-1 (nuclear respiratory factor 1) and NRF-2, as well as ERR-α (estrogen-related receptor-α) [43,44]. They subsequently induce the expression of TFAM (mitochondrial transcription factor A). TFAM is crucial for mtDNA replication and transcription, ensuring proper mitochondrial function. In human cells, mitochondria comprise roughly 1500 proteins, yet only 13 of these are encoded by their circular mtDNA (~16.5 kb), which is transcribed into RNA and then translated into proteins within the organelle. In addition to encoding twenty-two tRNAs and two rRNAs [45], mtDNA depends on TFAM for its proper expression and maintenance. The dysfunction of TFAM has been linked to severe respiratory deficiencies and embryonic lethality. By binding to mtDNA promoters, TFAM induces a U-shaped turn that helps to assemble the transcription initiation complex with POLRMT (mitochondrial RNA polymerase) and TFB2M (transcription factor B2, mitochondrial), while also coating and compacting mtDNA into nucleoids to protect the genome and regulate its copy number [46]. The intricate interplay among mitochondrial dynamics, quality control, and gene expression is not only essential for cellular homeostasis but also critically influences lung pathophysiology.

3. Mitochondrial Function Shapes the Responsiveness of Lung Cells

Through their roles in controlling cellular metabolism and maintaining homeostasis, mitochondria are key to sustaining the lung’s fine architecture. Mitochondrial activity and positioning are critical for alveolar development, particularly during secondary septation. In Sox9^Cre/+; Tfam^flox/flox and Sox9^Cre/+; Rhot1^flox/flox mice models, disrupting TFAM or RHOT1/MIRO1 in mouse alveolar epithelial cells reduces ligand secretion (e.g., PDGF), impairs myofibroblast growth and contractility, and hinders proper alveolar structure. Likewise, tampering with mitochondrial dynamics in fibroblasts (Pdgfra^Cre/+; Tfaml^flox/flox) compromises their cytoskeleton, affecting contraction and migration [47]. The study further connects these defects to emphysema, where the mitochondrial DNA copy number and TFAM protein are diminished. Overall, mitochondria play roles beyond ATP production, orchestrating lung cell behavior and providing potential therapeutic targets for alveolar disorders.

Mitochondria synthesize ATP through oxidative phosphorylation, a process driven by five respiratory chain complexes in the inner mitochondrial membrane. The largest among these, NADH ubiquinone reductase (complex I), exhibits a distinctive L-shaped structure. As a redox-driven proton pump, complex I initiates the electron transport chain by accepting electrons from NADH and transferring them to quinone [48]. Recent evidence highlights the critical role played by mitochondrial complex I-dependent NAD⁺ regeneration in postnatal alveolar development. In mice, deleting the complex I subunit Ndufs2 (NADH dehydrogenase (ubiquinone) iron–sulfur protein 2) in common lung epithelial progenitor cells during gestation (NDUFS2 cKO; SFTPC^Cre; Ndufs2^flox/−; ROSA26Sor^CAG-tdTomato) resulted in postnatal lethality and the emergence of hypertrophic transitional cells co-expressing AT1 and AT2 markers. The alveoli of NDUFS2 cKO lungs exhibited hypercellular regions with thickened walls interspersed among enlarged alveolar spaces. Remarkably, the conditional expression of the yeast NADH dehydrogenase NDI1, which regenerates NAD⁺ without proton pumping, rescued normal alveolar development and survival, suggesting that the inhibition of NAD⁺ regeneration is the key driver of the abnormality. These transitional cells exhibited heightened integrated stress response (ISR) gene expression; both small-molecule ISR inhibitors targeting eIF2α phosphorylation (ISRIB) and NAD⁺ precursor supplementation reduced this response and partially improved survival. Notably, the loss of the complex II subunit Sdhd (succinate dehydrogenase complex subunit D), despite its role in NAD⁺ regeneration, did not trigger similar ISR activation, underscoring the unique function of complex I in directing alveolar epithelial cell fate by preventing pathological ISR induction [49].

LONP1 is a serine peptidase that homo-oligomerizes to form a soluble hexameric ring for binding and cleaving varied substrates in the mitochondria matrix [50]. LONP1 is a critical regulator of airway epithelial homeostasis, orchestrating the balance between progenitor and differentiated cells. The conditional Lonp1 knockout mouse models (Shh^Cre/+; Lonp1^flox/flox, Sox2^CreERT2/+; Lonp1^flox/flox) show that the loss of LONP1 impairs mitochondrial proteostasis—evidenced by reduced electron transport chain (ETC) components and increased reactive oxygen species—in turn activating the integrated ISR via the PERK-eIF2a-ATF4 cascade. This mitochondrial dysfunction results in diminished progenitor proliferation and differentiation resembling COPD pathology, including the loss of club and ciliated cells, while basal and goblet cell populations expand abnormally. Furthermore, the pro-apoptotic protein BOK mediates the connection between mitochondrial deficiency and apoptosis induced via ISR in ciliated cells. Both pharmacologic (using ISRIB) and genetic interventions targeting the ISR rescued these cellular defects. These findings underscore the importance of mitochondrial integrity in directing airway cell fate and illuminate potential molecular mechanisms underlying chronic lung disease [51].

Notably, both studies observe that alterations in the ISR are central to their respective findings. ISR is a conserved pathway that converges on the phosphorylation of the translation initiation factor eIF2α to reduce global protein synthesis while boosting the production of stress-adaptive proteins [52]. It is activated by four main kinases—PERK, PKR, GCN2, and HRI—in response to diverse challenges such as endoplasmic reticulum (ER) stress, viral infection, amino acid deprivation, and oxidative damage. By selectively translating factors such as ATF4, the ISR helps cells to adapt metabolically and restore proteostasis, but chronic activation can trigger apoptosis via CHOP and other effectors. This balance between protective and pro-apoptotic outputs makes the ISR highly relevant to diseases including neurodegeneration, cancer, and diabetes, spurring the development of inhibitors (e.g., ISRIB). Given its profound involvement in the fate determination of lung cells, the mitochondria-associated ISR pathway may represent a pivotal target for developing innovative therapeutic interventions across a spectrum of pulmonary disorders.

4. Chronic Obstructive Pulmonary Disease (COPD)

COPD is a major global health problem predominantly caused by tobacco smoke and environmental pollutants. Additional genetic, developmental, and socioeconomic factors contribute to its onset and progression. In 2017, about 55% of chronic respiratory disease cases were attributed to COPD. By 2021, COPD had become the fourth leading cause of death globally [53]. Characterized by progressive and persistent respiratory symptoms, notably chronic cough with sputum production and exertional dyspnea, COPD often begins in midlife and can remain unrecognized until significant lung damage has occurred, highlighting the importance of early detection. Exposure to tobacco smoke, other pollutants, and smoking results in airway injury. This injury is clinically manifested by sputum hypersecretion, difficulty breathing, and an increased risk of acute exacerbations during which respiratory symptoms suddenly worsen. The diagnosis and management of COPD emphasize early detection (via spirometry) and a combination of non-pharmacological and pharmacological interventions. Smoking cessation, pulmonary rehabilitation, and vaccination remain foundational; home-based or virtual programs expand access when in-person care is limited. Treatment centers on inhaled bronchodilators (LAMA, LABA) and inhaled corticosteroids (ICS) and is guided by symptom burden, exacerbation risk, and—when appropriate—blood eosinophil levels. Triple therapy (ICS/LABA/LAMA) benefits individuals with frequent exacerbations, and prophylactic regimens (e.g., azithromycin, roflumilast), alongside robust self-management, can further reduce exacerbations and improve outcomes [54].

Exposure to CSE (cigarette smoke extract) is primarily used in in vitro experimental systems as a representative method to investigate the effects of cigarette smoke on cells. Long-term CSE (6 months, 10% CSE) in human bronchial epithelial cells of BEAS-2B induces marked and lasting mitochondrial alterations, including increased fragmentation, branching, matrix density, and decreased cristae numbers—most of which persist even after CSE withdrawal [55]. These morphological changes include elevated OPA1 mRNA levels, increased expression of OXPHOS proteins, and heightened oxidative stress (evidenced by elevated Mn-SOD), suggesting greater energy demand. In parallel, pro-inflammatory cytokines (IL-1β, IL-6, IL-8) are upregulated in long-term CSE-treated cells, reinforcing the link between mitochondrial dysfunction and inflammation. Notably, ex-smoking COPD patients exhibit similar mitochondrial abnormalities in their bronchial epithelia—cristae depletion, increased branching, elongation, and swelling, along with increased Mn-SOD, PINK1, PPARGC1α, and F1 α-subunit of ATP synthase—mirroring the chronic impact of CSE on mitochondrial structure and function, despite ceasing smoking. These observations highlight a persistent mitochondrial-driven inflammatory phenotype in COPD, underlining the potential therapeutic value of targeting mitochondrial pathways.

Furthermore, the short-term (24 h) CSE treatment of alveolar epithelial cells—both MLE12 (10 and 25% CSE) and primary mouse cells (50% CSE)—upregulates the mitochondrial fusion protein mitofusin 2 (MFN2) and promotes mitochondrial hyperfusion. This elongation coincides with enhanced mitochondrial function, reflected by increased metabolic activity and an elevated membrane potential in response to CSE treatment [56]. Another study demonstrated the short-term CSE (24 h, 10% CSE) exposure of BEAS-2B cell-induced mitochondrial enlargement and elongation, accompanied by a reduction in mitochondrial membrane potential and decreased protein levels of OPA1, MFN1, and MFN2. Furthermore, MFN1 and MFN2 levels were decreased in the lung homogenates of COPD patients [57]. These divergent findings may reflect variability in CSE concentration, preparation protocols, or the use of distinct cell lines (MLE12 and BEAS-2B).

OPA1-associated cristae modulators are altered in COPD lungs. Short OPA1 isoforms and mitochondrial stress-associated protein SLP2 (stomatin-like protein 2) were found to significantly increase in COPD lungs. In vitro, acute CSE treatment (4 h, 2% CSE) converts the long OPA1 isoform into its short counterpart in BEAS-2B cells. This CSE exposure also elevates SLP2 (stomatin-like protein 2). Meanwhile, the expression of OPA1-associated prohibitins (PHB1 and PHB2) varies depending on the degree of cigarette smoke exposure [6]. SLP2 is an SPFH superfamily member that localizes to the mitochondrial inner membrane to organize the cristae structure. SLP2 inhibits the stress-activated peptidase OMA1 (PHB1 and PHB2), which can bind to SLP2 and restrict the OMA1-mediated processing of OPA1, allowing stress-induced mitochondrial hyperfusion [58]. SLP2 interacts with MIC13, which is critical for maintaining the morphology of cristae and their junctions. Acting as a central hub for MICOS subunits, SLP2 stabilizes MIC26 by preventing its YME1L-mediated degradation. These findings position SLP2 as a key regulator of cristae architecture. The MICOS complex also forms a physical association with OPA1, which governs the number and stability of cristae junctions [39]. Notably, pharmacological interventions with BGP-15, a hydroxylamine derivative that activates OPA1, and leflunomide, an inhibitor of the mitochondrial enzyme DHODH (dihydroorotate dehydrogenase) that halts pyrimidine synthesis, effectively preserves the long OPA1 isoform under CSE challenge [6,7,59]. These findings underscore the pivotal importance of the long OPA1 isoform, alongside SLP2 and the prohibitin complex, in cigarette smoke-induced pulmonary injury. Their dysregulation contributes to mitochondrial dysfunction in COPD, highlighting a novel therapeutic avenue for mitigating disease progression.

Cell-free mtDNA has been recently highlighted as a hallmark of COPD [57]. The exposure of BEAS-2B bronchial epithelial cells to a sublethal dose of cigarette smoke extract (CSE) triggers the release of mtDNA via extracellular vesicles into the culture medium. Moreover, mice chronically exposed to cigarette smoke develop emphysema alongside elevated serum cell-free mtDNA. Former smokers with COPD harbor significantly higher levels of cell-free mtDNA compared to former smokers without airway obstruction [57]. The latter was confirmed in a bigger human cohort—the ECLIPSE study—in which circulating mtDNA was found to be higher in the plasma of COPD patients with mild and moderate COPD. This study predicted future exacerbations, suggesting that circulating mtDNA is a biomarker for COPD exacerbations [60]. Released mtDNA not only serves as a biomarker but also directly provokes inflammation. Extracellular mtDNA binds to the RAGE (receptor for advanced glycation end-products) and is internalized into endolysosomal compartments, where it activates TLR9 (toll-like receptor 9) [61]. The engagement of the RAGE–TLR9–MyD88–NF-κB signaling axis drives the production of pro-inflammatory cytokines, such as TNF-α and IL-6, in both immune and structural lung cells. Indeed, TLR9 expression is upregulated in smoke-exposed CD8⁺ T cells, and the genetic deletion of TLR9 protects mice from developing cigarette smoke-induced emphysema, underscoring its pivotal role in COPD pathogenesis. Moreover, mtDNA-driven, mitochondrial ROS-dependent NETosis (neutrophil extracellular trap-induced cell death) may be involved in COPD exacerbations. These insights highlight mtDNA-mediated RAGE–TLR9 signaling and NETosis as drivers of COPD and suggest that interventions such as mitochondrial antioxidants, DNase I, and TLR9 inhibitors may represent promising therapeutic strategies.

MitoQ is a mitochondria-targeted antioxidant formed by linking ubiquinol to a lipophilic cation. This design allows it to cross outer membranes and accumulate on the matrix-facing side of the inner mitochondrial membrane to reduce mtROS [8]. An in vitro study showed that MitoQ reduced mtROS and significantly attenuated CSE-induced PINK1 stabilization and Drp1 phosphorylation in Beas-2B cells [62]. In an ozone-exposed mouse model of COPD, MitoQ restored mitochondrial membrane potential and oxidative phosphorylation, reduced ROS levels, and alleviated both inflammation and airway hyper-responsiveness. In COPD patient-derived airway smooth muscle cells, MitoQ suppressed ROS and inflammatory mediator release while curbing cell proliferation. Thus, mitochondrial ROS targeting with MitoQ may be a promising strategy against COPD-related airway inflammation and remodeling [9].

In summary, COPD is caused by cigarette smoke damaging mitochondria, leading to oxidative stress and inflammation, and mitochondrial-targeted treatments, such as MitoQ, may help reduce these effects. Currently, the clinical trial NCT02966665 is rigorously evaluating the effects of MitoQ on vascular function in patients with COPD and related conditions. Building on such research, the applicability of mitochondria-targeting drugs for COPD is expected to be evaluated from a variety of perspectives. Interestingly, other studies found similar results in vitro and in vivo by expressing the alternative oxidase from the tunicate Ciona Intestinalis (AOX) in mice, which decreased mtROS produced through inhibiting mitochondrial respiratory chain transport upon exposure to cigarette smoke [63]. The lipofection of chemically modified, codon-optimized AOX mRNA (AOX cmRNA) into nondividing cells rescued their survival under complex III inhibition. Crucially, transient AOX cmRNA delivery restored respiratory bypass without genomic integration risks [64]. These findings highlight that AOX could also be potentially used to prevent/alleviate COPD.

5. Bronchopulmonary Dysplasia (BPD)

BPD is a leading complication of prematurity, affecting up to 45% of infants born before 29 weeks of gestation [65,66]. Despite advances that improve survival, its incidence has not declined. In parallel with progress in neonatal intensive care, BPD has shifted from a fibrocystic disease primarily affecting late preterm infants to a condition characterized by insufficient parenchymal development and dysregulated vascular growth. BPD was originally characterized by lung injury from mechanical ventilation and oxygen therapy in preterm infants born at 26–30 weeks of gestation. Advances such as maternal corticosteroids and surfactant replacement have shifted the disease phenotype to younger gestational ages (24–26 weeks of gestation). Consequently, “new” BPD is defined by the need for supplemental oxygen beyond 28 days or at 36 weeks of gestation and is marked by impaired alveolar and capillary development in immature lungs. BPD is a systemic condition with substantial implications for long-term health and quality of life, generating significant healthcare costs well beyond the initial hospitalization. The absence of an objective definition for predicting future morbidity and mortality underscores the complex, multifactorial nature of BPD, posing a major challenge to developing a universally accepted definition.

To what extent are mitochondria implicated in the pathogenesis of BPD? A recent pilot study examined extremely preterm infants with BPD, identifying mtDNA mutations in seven out of ten patients (21 point mutations overall). Many mutations were located in genes essential for respiratory chain complexes (e.g., MT-ND1, MT-ND6, MT-ND4, MT-ND5, MT-W, MT-ND2, MT-I, MT-ATPase6), suggesting a link between mitochondrial dysfunction and BPD pathogenesis. Further research is needed to clarify these mechanisms [67].

In animal models, exposure to supplemental oxygen alone or positive pressure ventilation alone is sufficient to induce a BPD phenotype characterized by impaired alveolar development and microvascular injury [68]. In neonatal rats exposed to hyperoxia (85% FiO₂), alveolar type II (ATII) cells exhibited mitochondrial fragmentation with increased DRP1/p-DRP1 and elevated glycolytic enzymes (PFKM, HK2, LDHA) alongside reduced ATP production, reshaping glucose metabolism and contributing to lung injury in BPD. Targeting DRP1 activation may, therefore, represent a promising therapeutic strategy. Further elucidating the molecular mechanisms underlying hyperoxia-associated damage will facilitate the development of future interventions to protect ATII cells in BPD models [69].

How significantly does mitochondrial metabolism affect BPD? Metabolomic analysis of plasma samples collected at 36 weeks post-menstrual age from infants with and without BPD identified significant metabolic alterations. Notable changes in TCA cycle-related metabolites, including cis-aconitic acid, itaconitic acid (ITA), and isocitric acid, were observed in infants with BPD. ITA is a metabolic product derived from cis-aconitic acid via a decarboxylation reaction in the TCA cycle, catalyzed by IRG1. In a rat model of BPD, administrating ITA improved lung morphology, promoted SP-C expression, and suppressed apoptosis in alveolar type II epithelial cells. From a mechanistic perspective, ITA enhances autophagic flux by promoting the nuclear translocation of transcription factor EB (TFEB), facilitating the clearance of damaged mitochondria, and reducing AEC II apoptosis. These findings uncover a pathway through which ITA counters hyperoxia-induced apoptosis, highlighting its promise as a potential therapeutic agent for BPD [70].

Mesenchymal stem cells (MSCs) can repair injured tissue by secreting immunomodulating paracrine factors to restore function and lung epithelial/endothelial integrity. The successful intranasal injection of human umbilical cord Wharton’s jelly MSCs restores alveolarization and vascularization in an experimental BPD mouse model, supporting the idea that MSCs contribute to repairing lung injuries in BPD patients [71]. The mitochondrial bioenergetics defects of MSCs are associated with a risk of BPD in extremely low birth weight infants (ELBWs). Umbilical cord-derived MSCs from ELBW infants who died or developed moderate/severe BPD exhibited mitochondrial dysfunction and increased oxidative stress, including lower oxygen consumption and aconitase activity, and higher extracellular acidification. Additionally, these cells showed reduced PINK1 and elevated TOM20 expression with more mitochondria per cell, suggesting a possible decrease in mitophagy. Furthermore, these MSCs had decreased proliferation and increased apoptosis compared to those from infants who survived with no or mild BPD. Mitochondrial dysfunction in MSCs from ELBW infants may offer key insights into the role of MSCs and the effectiveness of autologous MSC transfusions for treating BPD [72].

6. Pulmonary Arterial Hypertension (PAH)

PAH is a rare yet progressive condition characterized by the remodeling and narrowing of the pulmonary vasculature, leading to right heart failure and, ultimately, death. Despite the development of targeted therapies that have improved patient morbidity and mortality, significant challenges persist due to ongoing disability and high healthcare costs. In 2021, this condition claimed 22,000 lives worldwide, translating to an age-standardized mortality rate of 0.27 per 100,000 individuals. Although rare, PAH imposes a substantial global health burden and disproportionately affects women and older adults [73]. PAH results from genetic mutations—most notably in BMPR2 (Bone Morphogenetic Protein Receptor Type 2) and related pathways—and epigenetic alterations, including DNA methylation, histone deacetylation, and microRNA dysregulation, that impair endothelial function. Environmental exposures (such as appetite suppressants, methamphetamine, dasatinib, and certain infections) and mechanical stresses further promote inflammation and vascular remodeling, disrupting vasoregulatory signals (including endothelin-1, nitric oxide, and prostacyclin). Collectively, these factors lead to pulmonary vasoconstriction and right ventricular dysfunction [74,75]. Structural alterations in the pulmonary vascular wall are partly driven by metabolic reprogramming that resembles cancer. In particular, reducing mitochondrial glucose oxidation across pulmonary arterial cell types, including smooth muscle cells, endothelial cells, and fibroblasts, suppresses mitochondria-dependent apoptosis [76]. This inhibition is accompanied by a compensatory increase in glycolysis, which promotes pro-proliferative signaling and redirects carbon substrates toward the synthesis of cellular building blocks rather than their oxidation for energy production.

Experimentally, microvascular endothelial cells isolated from an SU5416/hypoxia rat model of PAH exhibit marked mitochondrial fragmentation associated with elevated mitochondrial reactive oxygen species (mtROS) and increased intracellular calcium. Excessive mtROS promotes calcium influx via the TRPV4 (Transient Receptor Potential Vanilloid 4) channel, leading to increased phosphorylation of Drp1 and consequent mitochondrial dysfunction characterized by a glycolytic metabolic shift and impaired OXPHOS. Notably, the pharmacological quenching of mtROS with MitoQ or inhibition of TRPV4 (HC-06704) effectively attenuates both mitochondrial fragmentation and its associated calcium dysregulation, suggesting that disrupting this feed-forward loop may help to restore normal mitochondrial and endothelial cell function in PAH [77].

Integrative transcriptomic and metabolomic analyses of PAH patient lung samples (164 PAH, 58 controls), combined with machine learning, reveal that in PAH, reduced electron transfer from cytochrome c to molecular oxygen and disrupted citric acid cycle homeostasis—evidenced by altered levels of 3-phenyllactic acid, ADP, and citric acid—may serve as key pathogenic mechanisms [78]. PAH patients show increased levels of PDK (pyruvate dehydrogenase kinase), which inhibits the mitochondrial enzyme PDH (pyruvate dehydrogenase)—a key regulator of glucose oxidation. The ex vivo treatment of PAH lung tissue with the PDK inhibitor dichloroacetate (DCA) activated PDH and enhanced mitochondrial respiration. In a subsequent 4-month open-label study, DCA treatment in idiopathic PAH patients already receiving approved therapies led to reductions in mean pulmonary arterial pressure and pulmonary vascular resistance, as well as improved functional capacity. These findings highlight PDK’s position as a viable therapeutic target, suggesting that genetic profiling may inform precision medicine approaches in treating PAH [10]. Considering the intricate molecular pathogenesis of PAH, exploring the role of mitochondria—mainly through epigenetic regulation—emerges as a promising direction for future research. Given that mitochondria-dependent metabolic processes underpin S-adenosylmethionine-dependent DNA methylation, mitochondria-mediated epigenetic regulation could serve as a novel mechanistic link between mitochondrial function and PAH [79].

7. Idiopathic Pulmonary Fibrosis (IPF)

IPF, the most common form of idiopathic interstitial pneumonia, is associated with the poorest prognosis, with a median survival of 2.5 to 3.5 years. This chronic, progressive interstitial lung disease has an unknown etiology and frequently affects older adults [80]. It typically presents with exertional dyspnea, a chronic dry cough, and Velcro-like crackles, yet it is frequently misdiagnosed or overlooked. Central to the disease process is the idea that repetitive, subclinical epithelial injury and accelerated cellular aging trigger abnormal alveolar repair and fibrosis. Various genetic and environmental factors further contribute to this process. High-resolution CT scanning is indispensable for diagnosis, particularly when a UIP (usual interstitial pneumonia) pattern is evident. Non-pharmacological management (e.g., smoking cessation, supplemental oxygen, pulmonary rehabilitation) and antifibrotic pharmacologic agents, namely, pirfenidone and nintedanib, can slow disease progression. Pirfenidone is thought to exert antifibrotic effects primarily through inhibiting collagen synthesis and modulating profibrotic signals, such as TGF-β (transforming growth factor beta). In contrast, nintedanib is a multikinase inhibitor that targets platelet-derived growth factor, fibroblast growth factor, and vascular endothelial growth factor pathways. Both agents have been shown in randomized controlled trials to reduce the annual decline in FVC (forced vital capacity) by roughly 50%, thereby extending the time to disease progression. However, each also carries side-effect profiles (notably gastrointestinal disturbances and potential hepatic injury) that mandate careful monitoring. Despite these therapeutic advances, IPF still carries a poor prognosis in older adults, and lung transplantation—though beneficial in select patients—remains an option for only a fraction of them. Nonetheless, recent insights into molecular underpinnings, such as the MUC5B (mucin 5B promoter variant) promoter variant and disruptions in immune pathways, offer promising therapeutic directions. Improved early recognition, refined diagnostic approaches, and continued exploration of novel antifibrotic and immunomodulatory strategies may ultimately transform IPF into a manageable chronic disease rather than a terminal one [81]. The circulating mitochondrial DNA (mtDNA) is a biomarker for predicting acute exacerbation and disease progression in idiopathic pulmonary fibrosis (IPF). By analyzing serum mtDNA levels in a cohort of IPF patients, the elevated mtDNA concentrations were associated with a higher incidence of acute exacerbation of idiopathic pulmonary fibrosis (AE-IPF), accelerated decline in lung function, and increased mortality. A threshold of 1371.5 copies/μL of mtDNA was identified as a reliable predictor of AE-IPF within one year, outperforming other biomarkers, such as KL-6 and SP-D. Circulating mtDNA could be a helpful prognostic tool in IPF, potentially guiding therapeutic strategies [82].

The mitochondria-mediated apoptosis of alveolar epithelial cells is increasingly recognized as a mechanism in IPF, including IPF. Repeated epithelial injury and accelerated cellular aging may compromise mitochondrial integrity, possibly triggering the release of pro-apoptotic cytochrome c and activating the apoptosis pathway [83]. This excessive epithelial cell death may promote aberrant repair and fibrotic remodeling in the lung. In IPF patient lungs, alveolar type II cells (AECIIs) accumulate dysfunctional, swollen mitochondria and exhibit upregulated ER stress markers—changes also seen in older mice under ER stress. These mitochondrial abnormalities are linked to the reduced expression of PINK1, a key regulator of mitochondrial quality control. Silencing PINK1 in lung epithelial cells triggers mitochondrial depolarization and profibrotic factor expression. Young PINK1-deficient mice develop similar mitochondrial dysfunction in AECIIs and are more prone to apoptosis and fibrosis. These findings suggest that PINK1 deficiency in IPF impairs mitophagy and, thereby, mitochondrial dysfunction and fibrotic changes [84]. PINK1 meets the E3 ubiquitin ligase PARKIN on the mitochondria outer membrane to trigger mitophagy. Similar to PINK1, PARKIN expression is reduced in IPF tissues, where fibroblastic foci are composed of fibrogenic myofibroblasts. Insufficient mitophagy regulated via PARKIN triggers an increase in reactive oxygen species and subsequently activates PDGFR/PI3K/AKT signaling. This pathway promotes myofibroblast differentiation and proliferation but can be reversed by antioxidants or PDGFR inhibition. Notably, PDGFR activation further suppresses autophagy, creating a self-amplifying loop. Moreover, PARKIN knockout mice exhibit enhanced bleomycin-induced fibrosis, which is alleviated by PDGFR inhibition. These findings indicate that reduced PARKIN-mediated mitophagy underlies IPF pathogenesis by driving the PDGFR/PI3K/AKT axis [85]. For mitophagy’s role in IPF, the PINK1–Parkin pathway has traditionally garnered significant attention. However, recent evidence underscores the physiological relevance of receptor-mediated mitophagy, and future studies are warranted to delineate its precise involvement in IPF pathogenesis.

Defects in mitochondrial dynamics and biogenesis also impair alveolar cell homeostasis. In murine AEC2 cells, the deletion of the MFN1 and MFN2 triggers spontaneous lung fibrosis and increases morbidity. Mechanistically, MFN1 and MFN2 are integral to the biosynthesis of surfactant lipids, orchestrating the synthesis of phospholipids and cholesterol to maintain alveolar integrity. Moreover, disruptions in this pathway, either through the absence of MFN1/MFN2 or by inhibiting lipid synthesis via fatty acid synthase deficiency, exacerbate bleomycin-induced lung fibrosis. These findings suggest a regulated interplay between mitochondrial fusion and lipid metabolism, offering novel insights into the molecular underpinnings of AEC2 injury and subsequent fibrotic remodeling in the lung [86]. The persistent repression of PGC1α in IPF fibroblasts establishes a pathological state characterized by diminished mitochondrial biogenesis, increased extracellular matrix production, and a pro-senescent secretory profile. PGC1α is stably repressed in IPF patient-derived fibroblasts. PGC1α levels are transiently reduced following bleomycin-induced injury in young mice, subsequently recovering during fibrosis resolution. In contrast, this recovery fails to occur in aged mice with persistent fibrosis. Furthermore, defects in PGC1α decrease mitochondrial mass and function in human lung fibroblasts. These defects also enhance contractile and matrix synthetic activation, upregulate senescence-associated genes, and intensify the release of profibrotic and pro-senescent signals. Conversely, the re-expression of PGC1α in IPF fibroblasts ameliorates these deleterious changes. Notably, pharmacological intervention with rosiglitazone—unlike thyroid hormone—effectively elevates PGC1α expression and attenuates fibroblast activation, highlighting the pivotal role of PGC1α in regulating fibroblast homeostasis and its potential as a therapeutic target in IPF [11].

How effective are mitochondrial-targeted drugs in treating IPF? The cell-permeable mitochondrial-targeted peptide SS-31 (H-D-Arg-Dmt-Lys-Phe-NH₂) is characterized by alternating aromatic and cationic residues. It accumulates in the inner mitochondrial membrane independent of mitochondrial potential. It effectively scavenges reactive oxygen species (ROS), preventing mitochondrial permeability transition, cytochrome c release, and oxidative cell death [12]. SS-31 (5 mg/kg) reduces lung fibrosis and inflammation in a bleomycin-induced mouse model of IPF by restoring mitochondrial function and activating Nrf2 signaling, inhibiting the NLRP3 inflammasome in macrophages. These effects were not observed in Nrf2-deficient mice, highlighting Nrf2′s key role and the possibility of SS-31 as a treatment for IPF [13]. A drug targeting the antiapoptotic factor Bcl-2 could be a potential candidate for treating IPF. MCU (mitochondrial calcium uniporter) drives metabolic reprogramming in lung macrophages, promoting pulmonary fibrosis through enhanced apoptosis resistance. In both IPF and bleomycin-injured lung macrophages, mitochondrial Bcl-2 is elevated via the increased activity of carnitine palmitoyltransferase 1a (Cpt1a), which binds to Bcl-2′s BH3 domain to anchor it in the mitochondria—a process suppressed in dominant negative-MCU mice. Consequently, deleting or inhibiting Bcl-2 (e.g., with ABT-199) protects against and even reverses established fibrosis, revealing a critical link between fatty acid β-oxidation, macrophage survival, and pathological tissue remodeling in IPF [14].

Metformin is widely recommended as the first-line therapy for type 2 diabetes mellitus (T2DM) in most clinical guidelines and is taken daily by more than 200 million people worldwide. Its popularity as a leading monotherapy option stems from its low cost, established safety, weight neutrality, and potential cardiovascular benefits. Metformin confers anti-inflammatory and immunomodulatory effects in immune-related diseases via AMPK (AMP-Activated Protein Kinase)-dependent and AMPK-independent pathways, targeting innate and adaptive immunity [15]. Since the early 2000s, mitochondria have been widely regarded as the principal organelles targeted by the glucose-lowering actions of metformin. This view is largely based on metformin’s accumulation within the mitochondria matrix and its specific, mild, and reversible inhibition of respiratory chain complex I. A recent report used cryo-electron microscopy and enzyme kinetics to reveal that metformin inhibits mammalian respiratory complex I through binding in an amphipathic region of the quinone-binding channel, as well as within a pocket on the intermembrane side. Metformin also induces an unexpected local “chaotropic” effect, displacing a segment of a crucial membrane domain helix [16]. Notably, metformin is gaining attention as a potential therapy for IPF. Metformin treatment in IPF lung myofibroblasts reduces fibrotic activity, enhances mitochondrial biogenesis, and restores apoptosis sensitivity. In a bleomycin-induced mouse model, metformin (65 mg kg^–1 i.p., every other day for 18 days) accelerates fibrosis resolution via AMPK. These findings highlight deficient AMPK activation in persistent fibrosis and support metformin’s role in reversing established fibrosis by facilitating myofibroblast deactivation and apoptosis [17]. In a US nationwide cohort study of 3599 patients with IPF and type 2 diabetes (T2DM), which utilized a large US healthcare database containing information on enrollees in private and Medicare Advantage health plans, a 1:1 propensity score-matched analysis (n = 2200) revealed that metformin therapy was associated with a significantly lower risk of all-cause mortality (hazard ratio [HR], 0.46; 95% confidence interval [CI], 0.36–0.58) and fewer hospitalizations (HR, 0.82; 95% CI, 0.72–0.93) compared to patients not receiving metformin [18]. These findings suggest that metformin may confer a clinical benefit in patients with both IPF and T2DM, further supporting the hypothesis that metformin’s antifibrotic properties could improve outcomes in this high-risk population.

8. Drug-Induced Lung Disease

Amiodarone, used for both supraventricular and ventricular arrhythmias, is a bi-iodinated benzofuran derivative that can accumulate in the lungs. The most serious side effect of pulmonary toxicity can occur at any dose and is associated with a relative risk of 1.77 compared to placebo. Commonly presenting as acute or subacute pneumonitis with diffuse infiltrates, other forms include pleural disease, migratory infiltrates, or nodules. Early detection improves outcomes, and stopping amiodarone plus corticosteroid therapy leads to recovery [87,88]. Amiodarone and its metabolite N-desethylamiodarone (DEA) were found to disrupt the mitochondrial membrane potential in alveolar macrophages, alveolar type II pneumocytes, and nonciliated bronchiolar epithelial (club) cells, significantly reducing ATP levels and cell viability. DEA acts more rapidly and potently than amiodarone. These findings suggest that the inhibition of mitochondrial respiration, particularly the greater inhibition of complex II by DEA, initiates pulmonary toxicity independently of the mitochondrial permeability transition [89]. In both human platelets and HepG2 cells acutely exposed to amiodarone, the cell-permeable succinate prodrug NV118 significantly alleviates the respiratory deficit by enhancing succinate-supported mitochondrial oxygen consumption and restoring ATP-generating capacity [90]. This study raises the possibility that, even in pulmonary cells, mitochondrial complex II might constitute a viable candidate target for mitigating amiodarone-induced toxicity. In amiodarone-induced pulmonary toxicity, mitochondrial dysfunction is an early event. In human peripheral lung epithelial HPL1A cells, 100 μM amiodarone caused a loss of mitochondrial membrane potential and cytochrome c release within 2 h, followed by ROS increase after 6 h, triggering cell death. Mitochondria-targeted antioxidants, such as coenzyme Q10 and α-tocopherol, were protective, whereas direct ROS scavengers were less effective, highlighting the importance of preserving mitochondrial integrity to prevent toxicity [19].

Because NSAIDs (non-steroidal anti-inflammatory drugs) effectively relieve pain and inflammation, they are widely used and appear on the WHO’s Model List of Essential Medicines [91]. However, many placebo-controlled trials and meta-analyses warn that NSAIDs can cause serious side effects in the digestive system, heart, liver, kidneys, brain, and lungs. Aspirin, the oldest NSAID, can sometimes provoke severe asthma-like symptoms known as aspirin-exacerbated respiratory disease (AERD). The prevalence of AERD is around 7% in typical adult asthma patients and roughly twice as high among those with severe asthma, underlining the importance of early recognition [92]. In understanding the mechanisms of NSAID action, several perspectives classify their effects based on major subcellular targets. Among these, PGHS (prostaglandin-endoperoxide synthase)-dependent and -independent pathways are the most widely recognized. The PGHS-dependent pathway involves modulating the production and abundance of prostanoids—key inflammatory mediators—to regulate tissue inflammation. In contrast, the PGHS-independent pathway centers on the cytotoxic effects of NSAIDs, particularly on mitochondria [91]. NSAIDs disrupt mitochondrial function and are linked to oxidative stress, the uncoupling of OXPHOS, the inhibition of the ETC, and mitochondrial depolarization. NSAIDs inhibit complex I activity in isolated mitochondria and Caco-2 cells. Interestingly, the addition of coenzyme Q and quercetin, a flavonoid that exerts a redox interaction with the ETC, restored complex I activity in [93]. The NSAID indomethacin activates the PKCδ–p38 MAPK–DRP1 signaling pathway, driving excessive mitochondrial fission and dysfunction that culminates in apoptosis. Suppressing DRP1 (via knockdown) or inhibiting p38 (with SB203580) mitigates these indomethacin-induced effects in human gastric carcinoma cells. In rat gastric mucosa, indomethacin promotes DRP1′s fissogenic activity and mitochondrial recruitment while downregulating fusogenic OPA1 and mitofusins. OPA1 deficiency disrupts cristae architecture, which explains the cristae deformities observed through electron microscopy [94]. The role of NSAID-induced mitochondrial defects in lung pathology has not been well characterized. However, compromised mitochondrial function may contribute to heightened oxidative stress and inflammation in pulmonary tissues [95].

Bleomycin (C₅₅H₈₄N₁₇O₂₁S₃) is an antibiotic anticancer drug approved by the FDA that is naturally synthesized by the Gram-positive bacterium Streptomyces verticillus. Bleomycin is commonly used to treat Hodgkin’s and non-Hodgkin’s lymphoma, testicular cancer, ovarian cancer, and cervical cancer. It can be prescribed alone or in combination with other treatments (e.g., doxorubicin for Hodgkin’s lymphoma, etoposide and cisplatin for sarcoma) and may also be combined with radiotherapy or immunotherapy (such as for Hodgkin’s lymphoma and melanoma) [96,97]. Bleomycin (1.28 U/rat)-induced lung injury is driven by interlinked mechanisms involving direct reactive oxygen species (ROS) production, cytokine and growth factor release, epigenetic changes, and epithelial–mesenchymal transition (EMT). Complex formation between bleomycin, ferrous ions, and water leads to ROS generation (e.g., hydroxyl radicals), triggering alveolar epithelial cell death and subsequent inflammation and fibrosis. DAMPs released from necrotic cells further activate toll-like receptors (TLRs), promoting pro-inflammatory and profibrotic responses through the NF-κB, MAPKs, and TGF-β pathways [96]. Furthermore, the intratracheal bleomycin administration model in rodents is frequently used to study lung fibrosis. Bleomycin administration causes nuclear and mitochondrial DNA damage, leading to respiratory chain dysfunction; early ROS production then triggers mtDNA deletions that correlate with increased TGFβ1 levels and ultimately result in lung fibrosis [98]. Therefore, to what extent do mtDNA alterations in fibroblasts contribute to bleomycin-induced lung fibrosis? In a novel murine model, the fibroblast-specific induction of a dominant-negative mutant of the mitochondrial helicase Twinkle (TwinkleFIBRO) led to 25% mtDNA depletion without disrupting respiratory chain stoichiometry. Rather than causing a classical respiratory defect, these fibroblasts exhibited diminished ROS production, lower mitochondrial membrane potential, and an anti-inflammatory/antifibrotic phenotype. This contrasted starkly with wildtype fibroblasts treated with sodium azide—an inhibitor of the cytochrome c oxidase—that displayed a pro-inflammatory and profibrotic signature. In vivo, upon the accumulation of mtDNA mutations, TwinkleFIBRO mice were protected against dermal fibrosis induced by bleomycin, reducing the differentiation of dermal fibroblasts into myofibroblasts. Together, these findings reveal an unexpected divergence between mtDNA mutation-induced mitochondrial dysfunction and pharmacologically blocked respiratory function [99].

Recent studies have revealed significantly downregulated fatty acid synthase (FASN) in bleomycin-treated mouse models. FASN, essential for producing palmitate and other fatty acids, appears to play a protective role in alveolar epithelial cells. Its overexpression markedly reduces bleomycin-induced AEC death, preserves mitochondrial membrane potential, and lowers the production of mitochondrial ROS. Conversely, FASN knockdown exacerbates cell death, underscoring its importance in cellular survival. Notably, the increase in oleic acid upregulated by FASN overexpression contributes to the mitigation of cell death and lung injury, as evidenced by reduced inflammation and collagen deposition in FASN transgenic mice. These findings collectively suggest that impaired FASN production may contribute to bleomycin-induced pathogenesis through mitochondrial dysfunction. Strategies aimed at enhancing FASN expression could hold therapeutic promise for preventing lung fibrosis [100]. Recent reports have shown that a bleomycin-induced pulmonary fibrosis model can be valuable for developing new treatment strategies, such as mitochondrial transfer, to combat lung-related disorders. One study described a mitochondria transfer system—Mito-MEN—that combines isolated healthy mitochondria with nanoparticles loaded with Parkin mRNA. This system enhances the delivery of functional mitochondria and stimulates mitophagy to clear damaged mitochondria. In vitro and in vivo experiments, including the bleomycin-induced pulmonary fibrosis model, have demonstrated that Mito-MEN effectively restored mitochondrial function, reduced fibrotic markers, and improved respiratory function. These findings suggest a promising approach for treating fibrotic and mitochondrial-related diseases [20].

9. Conclusions

Across the spectrum of lung diseases, the central role of mitochondria in maintaining cellular homeostasis has become increasingly evident. Molecular and cellular biology, physiology, and pharmacology approaches have revealed that the complex balance of mitochondrial dynamics, quality control, and metabolic reprogramming determines cell fate. It profoundly influences tissue repair and the progression of lung pathology. In diverse pulmonary disorders, disruptions of mitochondrial fission and fusion balance, mitophagy, and mitochondrial biogenesis are linked to oxidative stress, chronic inflammation, and fibrotic remodeling. These molecular insights pave the way for innovative therapeutic strategies targeting mitochondrial dysfunction. These strategies include antioxidants; targeted modulation of respiratory chain complexes (for example, using complex I inhibitors, such as metformin); metabolic modulators; gene therapy approaches; and mitochondrial transfer technology. Nonetheless, implementing these discoveries in clinical settings remains a significant hurdle. Future research should focus on further identifying robust mitochondrial biomarkers for early detection and risk stratification through pathological approaches. At the same time, well-designed clinical trials are essential to evaluate the efficacy and safety of mitochondrial-targeted therapies. Mitochondria are, of course, essential for the function of normal tissues, so it is crucial that therapies selectively target pathological tissue and molecules to prevent unintended side effects. Moreover, a more resonant exploration of the interplay between mitochondrial dysfunction and the immune system may produce further insights into the mechanisms driving chronic inflammation and fibrosis. By combining these fundamental insights with clinical innovations, we will be at the threshold of a new era in lung disease management. Restoring mitochondrial integrity could significantly improve patient outcomes and reduce the global burden of respiratory disorders.