Controlled Cell Death Processes in COPD: How They Work and Possible Treatments

A distinct form of programmed cell death, or apoptosis, as coined by Kerr, was first introduced in 1972 [17]. The hallmark morphological changes in apoptosis include cell rounding, nuclear condensation, plasma membrane blebbing, and the formation of apoptotic bodies [18]. Furthermore, it can be categorized into intrinsic apoptosis and extrinsic apoptosis [19] (Figure 2). Intrinsic apoptosis is initiated with mitochondrial outer membrane permeabilization (MOMP), which releases cytochrome c (Cyto-C) into the cytoplasm. In the cytoplasm, Cyto-C interacts with Apaf-1 to form the apoptosome, which subsequently activates caspase-9. This initiator caspase then triggers the caspases-3/7, leading to cellular disintegration [20]. Notably, this classical pathway is tightly regulated by intrinsic inhibitory factors. For example, anti-apoptotic Bcl-2 family proteins (Bcl-2, Bcl-xL, and Mcl-1) are capable of preventing MOMP by directly binding and inhibiting Bax/Bak [21], while mitochondrial fusion protein Mfn2 has been found to inhibit Bax activation through direct interaction [22]. Extrinsic apoptosis is triggered by the binding of a specific ligand to its corresponding receptor (Fas and TNF receptor superfamily member 1A (TNFRSF1A)) on the cell surface. Following ligand binding, the death-inducing signal complex (DISC) is assembled through the clustering or trimerizing of the death receptors, which recruit Fas-associated death domain (FADD) together with procaspase-8/10. To increase the apoptotic signal, the activated caspase-8/10 either directly or indirectly cleaves Bid to produce tBid, which in turn activates Bax/Bak and promotes MOMP [23]. Moreover, caspase-9 or death-associated protein kinase 1 (DAPK1) can be activated by receptor-dependent pathways involving Unc-5 Netrin Receptor B (UNC5B) and DCC Netrin 1 Receptor (DCC) after dephosphorylation induced by ligand withdrawal [18,24]. In all cases, the caspase cascade is crucial to the successful completion of apoptosis [25].

The percentage of apoptotic cells in the lung parenchymal cells of patients with COPD is approximately 1–5%, which is more than twice the rate observed in healthy lungs [26]. Chen et al. [27] demonstrated that apoptosis is markedly increased in both bronchial and alveolar epithelial cells in emphysema mouse models generated by exposure to cigarette smoke (CS) or intraperitoneal administration of cigarette smoke extract (CSE). In addition, widespread apoptosis of pulmonary vascular endothelial cells has been noted in COPD, with evidence suggesting that this process not only represents a key pathological feature but may also actively drive disease pathogenesis [28,29]. Importantly, the ability of alveolar macrophages to phagocytize apoptotic epithelial cells is compromised in COPD, due to both intrinsic defects in macrophage function and excessive apoptotic burden. The resulting persistence of uncleared apoptotic cells results in secondary necrosis, thereby exacerbating inflammation and accelerating COPD progression [26,30]. The regulatory network governing apoptosis in COPD is complex and involves multiple intracellular signaling pathways [31]. Sonic hedgehog (Shh) signaling pathway significantly participates in the maintenance of pulmonary cellular homeostasis and suppressing apoptosis, and its dysfunction has been closely linked to COPD development [32]. Li et al. [33] reported that CSE stimulation downregulates Shh pathway-associated protein expression in MLE cells. Treatment with recombinant mouse Shh protein partially reversed apoptosis, whereas inhibition of Shh signaling with cyclosporine A (an inhibitor of the Shh pathway) enhanced apoptosis in MLE-12 cells. Furthermore, apoptosis in COPD undergoes precise regulation via multiple epigenetic mechanisms. Fu et al. [34] demonstrated that miR-613 exacerbates CSE-induced bronchial epithelial cell apoptosis and epithelial–mesenchymal transition (EMT) through the targeting of ARHGAP1, thereby contributing to inflammatory responses and airway remodeling in COPD. Similarly, Li et al. [35] showed that CSE reduces PRMT6 expression and H3R2me2a levels in pulmonary epithelial cells while upregulating Bax expression and inflammatory cytokine production. Inflammation, another central feature of COPD pathology, is also closely linked to apoptosis [10,36]. Patients with COPD present with higher levels of inflammatory mediators such as TNF-α and interferon-γ [37]. TNF-α is primarily produced by alveolar macrophages and mediates apoptosis by activating the extrinsic apoptotic pathway [38]. Interferon-γ, mainly secreted by Th1 CD4⁺ and CD8⁺ T lymphocytes, induces apoptosis in type II alveolar epithelial cells [39]. These inflammatory mediators further recruit neutrophils, lymphocytes, and macrophages to the lung parenchyma and airways. The accumulation of macrophages and neutrophils leads to protease imbalance and oxidative stress, whereas adaptive immune cells undergo proliferative expansion and promote apoptosis of alveolar epithelial cells [40].

Accumulating evidence indicates that CS can induce endoplasmic reticulum stress, resulting in the activation of the unfolded protein response (UPR), disrupts protein homeostasis, and ultimately triggers apoptosis [41]. Feng et al. [42] found that CSE promotes apoptosis in Raw264.7 macrophages by upregulating ER stress-related proteins and activating the Ca²⁺/P38/STAT1 signaling pathways, suggesting that CS-induced macrophage dysfunction may increase susceptibility to pulmonary infections in COPD. Furthermore, Chen et al. [43] demonstrated in human airway smooth muscle cells (HASMCs) that CSE upregulates ORMDL3, leading to ATF6 activation, disruption of endoplasmic reticulum calcium homeostasis, and enhanced secretion of inflammatory cytokines, thereby facilitating airway remodeling, persistent inflammation, and airflow limitation in COPD.

Research on necroptosis commenced in 2005 [44]. Unlike apoptosis, necroptosis is caspase-independent and does not involve apoptotic body formation [45]. Instead, it is defined by the rupture of the plasma membrane, secretion of intracellular contents, and leakage of damage-associated molecular patterns (DAMPs), such as HMGB1, S100 proteins, and ATP, thereby triggering a potent inflammatory response [46]. Necroptosis is typically initiated through ligand-receptor signaling, whereby external or internal stimuli activate death receptors like TNFR1. Upon binding, TNF receptor-associated death domain protein (TRADD), TRAF2, RIPK1, and cIAPs are induced by TNFR1, thereby initiating NF-κB-mediated pro-survival signaling. Deubiquitination of RIPK1 promotes its release from complex I and subsequent formation of complex IIa with FADD and caspase-8, thereby initiating apoptosis. However, the inhibition of caspase-8 activity activates RIPK3, triggering the intracellular kinase cascade activation further [47]. Phosphorylated RIPK1 cooperates with RIPK3 and MLKL to form the necrosome, within which RIPK3 and MLKL are sequentially phosphorylated. Activated MLKL then oligomerises and inserts into the plasma membrane, generating disruptive pores that compromise membrane integrity and execute necroptosis [48,49] (Figure 3).

Necroptosis is strongly upregulated in the lungs of patients with COPD and contributes to airway inflammation, airway remodeling, and emphysema formation [50]. Relevant studies have shown that MLKL expression in alveolar epithelial cells and macrophages of patients with severe disease is elevated, accompanied by increased levels of phosphorylated RIPK3(p-RIPK3) and phosphorylated MLKL(p-MLKL) in lung tissue [50]. Conversely, the absence of RIPK3 or MLKL reduces CS-induced macrophage death, thereby alleviating airway inflammation, remodeling, and the pathological progression of emphysema [51]. Furthermore, RIPK1 deficiency diminishes smoking-induced pulmonary cell apoptosis and necrotic cell death, thereby alleviating airway inflammation, remodeling, and emphysema [52] while also preventing cigarette-induced impairment of mucociliary clearance [53]. The specific mechanisms by which necroptosis contributes to COPD pathogenesis may include aberrant upregulation of pro-inflammatory mediators, inefficient removal of dead cells, and promoting oxidative stress. For example, Zeng et al. [54] demonstrated that miR-21 knockout reduces necrotic apoptosis-associated proteins (TNFR1 and p-MLKL) in bronchial epithelial cells, concurrently lowering inflammatory cytokine levels and thereby ameliorating COPD symptoms. According to Wang and colleagues [55], GRP78 facilitates CSE-induced inflammation of airway epithelial cells and excessive mucus production by upregulating necrotic apoptosis and subsequently activating NF-κB and AP-1 pathways. In addition, Chen and associates [56] showed that impaired macrophage clearance of necrotic cells and DAMPs reduces phagocytic efficiency, thereby amplifying pro-inflammatory factor expression and ultimately triggering pathological inflammatory responses. Another study further reported that CSE induces necroptosis in bronchial epithelial cells, triggering the secretion of mitochondrial DNA (mtDNA) into the cytoplasm and extracellular space, which in turn activates inflammatory signaling [57].

Ferroptosis is a controlled type of RCD that is initiated by oxidative disturbances within the intracellular microenvironment and defined by its reliance on iron and lipid peroxidation. Morphologically, it is characterized by mitochondrial shrinkage, increased membrane density, decreased cristae density, and eventual rupture of the mitochondrial membrane [58]. Its mechanisms are complex and involve interactions among iron metabolism, glutathione metabolism, and lipid peroxidation (Figure 4). In iron metabolism, Ferric iron (Fe³⁺) is taken up by cells via transferrin receptors (TFRs), where it is converted to ferrous iron (Fe²⁺) within endosomes, and subsequently transported into the labile intracellular iron pool. Under pathological conditions such as infection or inflammation, disruption in iron homeostasis may result in an accumulation of iron within cells. Excess iron accelerates lipid peroxidation of polyunsaturated fatty acids (PUFAs), ultimately driving ferroptosis. The cystine/glutamate antiporter system Xc⁻ is responsible for maintaining redox balance. This system consists of the light chain subunit xCT (SLC7A11) and the heavy chain subunit CD98hc (SLC3A2), which facilitates the import of cystine in exchange for glutamate [59]. Intracellular cystine is enzymatically converted into glutathione (GSH), which, as an electron donor, reduces phospholipid hydroperoxides (PL-OOH) to their corresponding alcohols through the action of glutathione peroxidase 4 (GPX4), thereby mitigating lipid toxicity and inhibiting ferroptosis [60]. Ferroptosis is a process that takes place via an iron-catalyzed lipid peroxidation mechanism. This process can be triggered by both non-enzymatic (Fenton reaction) and enzymatic mechanisms (lipoxygenases, ALOXs). The Fenton reaction, mediated by Fe²⁺, produces reactive oxygen species (ROS) such as hydroxyl radicals. These ROS engage in lipid peroxidation reactions with phospholipid-containing PUFAs to generate lipid peroxidation products (PL-PUFA-OOH) such as malondialdehyde (MDA) and 4-hydroxynonenal (HNE), which disrupt cell membrane fluidity and permeability, leading to cell death. In parallel, ALOXs, a family of non-heme iron-containing enzymes, catalyze the PUFA oxygenation to produce PL-PUFA-OOH, further amplifying lipid peroxidation and promoting ferroptosis.

In 2012, B. Stockwell introduced the concept of ferroptosis [61] and was implicated in the regulation of COPD formation in 2019 [62]. Growing evidence reveals that patients with COPD display noticeably raised levels of ACSL4, LPCAT3, and lipid peroxidation products compared to healthy controls [63]. CS increases the expression of the key ferroptosis protein NCOA4 in COPD, promoting epithelial ferroptosis, iron release, and iron accumulation. Excess intracellular iron enhances ROS production through the Fenton reaction, further depleting GPX4, exacerbating lipid peroxidation, and inducing cell death, airway inflammation, airway remodeling, and emphysema development [64]. Ferroptosis is therefore characterized by iron overload, excessive lipid peroxidation, and disruption of the system Xc⁻/GSH/GPX4 antioxidant axis [65], collectively driving cellular damage and contributing to COPD pathogenesis.

As previously mentioned, ACSL4 is pivotal in lipid peroxidation and acts as a crucial regulator of ferroptosis. Wang et al. [66] reported that both Lv-shACSL4-treated COPD mouse models and siACSL4-transfected cells effectively suppressed ACSL4 expression, decreased lipid peroxidation, and downregulated ferroptosis markers in vivo and in vitro. This suggests that ACSL4 is a key factor in ferroptosis in COPD. Liu and colleagues [67] found that the deubiquitinating enzyme USP8 was downregulated in COPD models. Its overexpression activated the OTUB1/SLC7A11 pathway, enhancing cystine uptake and GSH synthesis to maintain GPx4 activity. This, in turn, suppressed ACSL4 expression, alleviating lung inflammation and airway remodeling. Epigenetic regulation influences the pathogenesis of COPD by affecting ferroptosis, offering new therapeutic directions. TET2, a DNA demethylase, catalyzes cytosine hydroxymethylation and chromatin remodeling to regulate gene expression, playing a crucial epigenetic role in myeloid hematopoiesis, inflammatory responses, and various pulmonary diseases. Zeng [68] et al. found that TET2 downregulation induces hypermethylation of the GPx4 promoter, impairing antioxidant defenses and exacerbating ferroptosis. Notably, co-treatment with the demethylating agent 5-AZA and the antioxidant NAC synergistically reduced ferroptosis and lung injury, suggesting that combining methylation inhibitors with antioxidants could provide a potentially new therapeutic approach for COPD. Xia et al. [64] identified the m6A-modified circSAV1/YTHDF1/IREB2 axis as the key driver of CS-induced ferroptosis in lung epithelial cells. This pathway promotes IREB2 mRNA translation, disrupts iron homeostasis, and exacerbates lipid peroxidation. Targeting circSAV1 or inhibiting key molecules in this axis significantly mitigated alveolar enlargement and airway remodeling in COPD mouse models.

Christian de Duve termed his observation of the lysosome-dependent degradation in cells, autophagy, in 1963 [69]. Autophagy is a highly conserved, pivotal biological process in eukaryotic cells that involves the breakdown of and recycling of damaged organelles, misfolded proteins, or invading pathogens. This process occurs within a double-membraned autophagosome, which fuses with lysosomes for degradation [70]. Autophagy-dependent cell death (ACD) represents a type of RCD governed by autophagic molecular processes. The Nomenclature Committee on Cell Death (NCCD) states that ACD is fundamentally different from classical types of cell death, like apoptosis and necrosis, and requires activation of the autophagy pathway or its key molecular components [19,71]. Based on substrate transport mechanisms, autophagy is primarily categorized into three major types: macroautophagy, microautophagy, and chaperone-mediated autophagy. Of these, macroautophagy represents the predominant form, commonly referred to simply as autophagy [72]. This process is strictly governed by a set of autophagy-related (ATG) proteins and occurs through five stages: induction, nucleation, elongation, maturation, and degradation (Figure 5). Under stress conditions, such as smoking, activation of AMPK or inhibition of mTORC1 leads to dissociation of mTORC1 from UNC-51-like kinase 1 (ULK1), resulting in ULK1 dephosphorylation and activation [73]. The activated ULK1 complex (ULK1-ATG13-FIP200-ATG101) subsequently recruits and activates the downstream Class-III PI3K complex (Beclin1-ATG14-VPS15-VPS34), which generates phosphatidylinositol 3-phosphate (PI3P) to initiate autophagosome nucleation [74]. Subsequently, microtubule-associated protein 1 light chain 3 (LC3) is cleaved by the ATG4 protease into LC3-I, which is then conjugated to phosphatidylethanolamine (PE) via the ATG12-ATG5-ATG16L1 ubiquitin-like conjugation system, forming membrane-bound LC3-II. LC3-II facilitates the extension and invagination of the phagophore membrane, ultimately enveloping substrates for degradation to form mature autophagosomes [75]. Autophagosomes are then fused with lysosomes to generate autolysosomes, which are responsible for the lysis of autophagosome contents using acidic hydrolases. The resulting metabolites, including amino acids and fatty acids, are recycled into cellular metabolic pathways, supplying both energy and biosynthetic precursors [76].

Accumulating evidence indicates that autophagy is dysregulated in COPD patients, exhibiting extremely dynamic and context-dependent bidirectional regulation. Compared to age-matched non-smokers, peripheral lung tissue from patients with severe COPD exhibits increased levels of p62, LC3, and protein aggregates, suggesting impaired autophagy in COPD [77]. Consistently, p62 accumulation in peripheral lung tissue positively correlates with pulmonary function severity and is closely associated with upregulation of LC3 and BICD1 expression [78]. In addition, alveolar macrophages from COPD patients and smokers display increased autophagosome formation, further supporting the notion that defective autophagic flux contributes to COPD pathogenesis [79]. Galectin-8 functions as a danger receptor, recognizing destroyed intracellular host vesicles that are damaged and recruits the autophagy adaptor protein NDP52 to facilitate forming and breaking down specific autophagosomes. Yuta Kono and associates [80] found lung homogenates from individuals with COPD demonstrated elevated galectin-8 and NDP52 levels, with significantly increased serum galectin-8 levels in those experiencing frequent acute exacerbations. At the cellular level, it has been demonstrated that CSE disrupts autophagic flux in U937 macrophage-like cells, resulting in the intracellular concentration of NDP52 and galactosidase-8. Conversely, chronic smoke exposure may also induce excessive autophagy activation, resulting in alveolar structural disruption, ciliary shortening, and impaired mucus clearance, thereby exacerbating airway remodeling and emphysema formation [81]. Bulk and single-cell RNA sequencing datasets from COPD and non-COPD tissues were examined by Liao et al. [82], who found significant upregulation of autophagy-related genes in COPD. Moreover, autophagy in macrophages and monocytes displayed extensive interactions with other cell types, highlighting its potential role as a key regulatory process in intercellular communication. Chen et al. [83] further demonstrated that mTOR deficiency enhances inflammatory responses, whereas autophagy inhibition via Atg5 deletion attenuates inflammation, suggesting that autophagy activation contributes to particulate aerosol-induced pulmonary inflammation.

Autophagy can be classified as either selective or non-selective forms based on the specificity of the degraded material [84]. Mitophagy, a selective type, specifically removes dysfunctional mitochondria to prevent ROS accumulation and apoptosis [85]. Abnormal activation of mitophagy is considered a key mechanism by which CS promotes COPD development. Yang et al. [86] demonstrated that ARSK alleviates COPD by inhibiting autophagy and restricting excessive mitophagy through suppression of Parkin Ser65 phosphorylation. Correspondingly, Wei and colleagues [87] revealed in a CS-induced mouse model that STAT3 activation enhances PINK1/Parkin pathway activity by upregulating TGF-β1, with excessive mitochondrial autophagy further inducing epithelial–mesenchymal transition (EMT) and small airway wall thickening. Beyond CS, environmental pollutants PM2.5 and microplastics also modulate mitochondrial autophagy through distinct mechanisms. Wang et al. [88] revealed that PM2.5 uncouples Klotho-mediated inhibition of IGF-1R, leading to Parkin-dependent mitophagy defects. Impaired clearance of damaged mitochondria subsequently triggers oxidative stress. Conversely, Wei et al. [89] demonstrated that polystyrene microplastics (PS-MPs) activate autophagy-dependent ferroptosis (ADF) via mitochondrial ROS accumulation, where NCOA4-mediated ferritinophagy releases free iron, exacerbating lipid peroxidation. These outcomes emphasize the complex and context-dependent function of mitochondrial autophagy in COPD. Insufficient mitophagy promotes oxidative stress, whereas excessive activation disrupts mitochondrial homeostasis. Despite operating through distinct mechanisms, both processes converge to drive airway remodeling and COPD progression [86].

The term "pyroptosis" was first introduced in 2005 [63]. Pyroptosis represents a type of RCD modulated by gasdermin (GSDM) proteins (Figure 6). It is structurally defined by chromatin condensation, cytoplasmic vesicle formation, and the rupturing of the plasma membrane. External factors like smoking and PM2.5 can trigger the release of pathogen-associated molecular patterns (PAMPs) or DAMPs. These molecules are recognized by pattern recognition receptors (PRRs), which trigger and induce the formation of inflammasomes [90,91,92,93,94]. Two major classes of PRRs involved in pyroptosis in COPD are nucleotide-binding domain leucine-rich repeat-containing proteins (NLRs) and AIM2-like receptors [95,96]. The formed inflammasome cleaves pro-caspase-1 into an active form, which then processes GSDM into the N-terminal fragment (GSDM-N) and matures pro-IL-1β and pro-IL-18. GSDM-N produces pores in the plasma membrane, enabling IL-1β/IL-18 release and amplifying inflammation [97,98,99]. In addition to this canonical pathway, pyroptosis can also occur through inflammasome-independent mechanisms. Intracellular lipopolysaccharide (LPS) activates caspase-4/5/11 directly, which then induces GSDMD cleavage and the release of GSDMD-N-terminal fragments [4,5].

Numerous chronic respiratory conditions, including COPD, have been linked to pyroptosis [100,101]. By 2013, studies demonstrated that IL-18 expression induces pulmonary inflammation, emphysematous changes, excessive mucus secretion, and fibrotic alterations in the blood vessels and airways of mice [102]. Nevertheless, the exact function of pyroptosis in COPD is still not fully determined. Faner and associates observed markedly elevated NLRP3, IL-1β, and IL-18 expressions in lung tissue from patients with stable COPD [103]. Subsequent studies further revealed that multiple pyroptosis-associated molecules, including NLRP3, caspase-1, and IL-18, are further upregulated during acute exacerbations of COPD [96,97]. Nicotine, a major constituent of CS, has been identified as a major driver of pyroptotic pathways [104]. Wang et al. [90] reported that nicotine exposure upregulates pyroptosis-associated molecules in 16HBE cells, potentially contributing to COPD progression. Similarly, Zhang et al. [105] demonstrated that CSE induces pyroptosis in 16HBE cells via the classical ROS/NLRP3/caspase-1 pathway, leading to GSDMD cleavage and release of IL-1β and IL-18. Rao et al. [106] further found that TRPV4 upregulation promotes pyroptosis by activating caspase-1 and increasing GSDM-N protein expression. Mechanistically, TRPV4 modulates the Ca²⁺/NLRP3/caspase-1 axis, while its inhibition attenuates NLRP3 inflammasome activation, lactate dehydrogenase release, and inflammatory cytokine production. While ferroptosis is defined by lipid peroxidation, lipid peroxidation can also initiate pyroptosis. Hou et al. [107] demonstrated that inhibiting xCT ubiquitination alleviates lipid peroxidation-mediated pyroptosis in COPD patients through downregulating the xCT/GSH/GPx4 axis. In vitro, Mo et al. [108] established a COPD cell model by treating the human lung fibroblast line MRC-5 with lipopolysaccharide and demonstrated that lncRNA GAS5 overexpression facilitates pyroptosis in pulmonary fibroblasts through activation of the miR-223-3p/NLRP3 axis. However, other studies yielded conflicting findings. One study reported that CSE suppresses NLRP3 inflammasome expression and its downstream cytokines IL-1β and IL-18 at the transcriptional level, while simultaneously activating caspase-1 via the noncanonical TLR4/TRIF/caspase-8 pathway [109]. This unique caspase-1 activation reduced glycolytic flux in macrophages, suggesting distinct and context-dependent cellular responses to CSE that require further investigation.

Cuproptosis (copper-induced cell death) denotes a recently identified form of programmed cell death, first described in 2022 by Tsvetkov and colleagues [110]. Its core mechanism involves the excessive binding of intracellular copper ions (Cu²⁺) to lipoylated proteins in the mitochondrial tricarboxylic acid (TCA) cycle, forming protein aggregation and destabilizing iron-sulfur (Fe-S) cluster proteins. This cascade induces proteotoxic stress, impairs mitochondrial respiration, and ultimately triggers cell death [111]. In contrast to other types of cell death, like ferroptosis and apoptosis, cuproptosis depends on copper–protein interactions and TCA cycle dysfunction, and cannot be blocked by conventional cell death inhibitors [112]. Evidence from COPD patients and cigarette smoke exposure models indicates significant dysregulation of cuproptosis-associated genes, including GLS, DLD, CDKN2A, and DLST. Among these, GLS expression is reduced and correlates with lung function decline, whereas DLD and CDKN2A are upregulated in response to smoking and related factors [113,114]. These genes not only serve as diagnostic biomarkers for COPD but are also implicated in copper homeostasis and cuproptosis-associated pathways, thereby offering potential avenues for targeted therapy [115]. Nevertheless, the exact mechanisms by which cuproptosis participates in the development of COPD pathogenesis remain unresolved, underscoring the need for further mechanistic studies and the development of targeted intervention strategies.

Parthanatos is classified as a type of RCD that is reliant on Poly(ADP-ribose) (PAR) polymerase-1 (PARP-1). PARP1 is a chromatin-associated nuclear protein that significantly participates in DNA repair during strand breaks [116,117,118]. Parthanatos can be induced by oxidative stress, such as that caused by smoking [119]. ROS can induce DNA damage, leading to the overactivation of PARP1 and the accumulation of PAR. This cascade promotes the nuclear translocation of mitochondrial proteins, including apoptosis-inducing factor (AIF) and Endonuclease G (EndoG). Once in the nucleus, AIF and EndoG cause chromatin dissolution and cell death [120]. Studies have shown that the EndoG inhibitor BMN673 can reduce levels of AIF and EndoG in epithelial cells [121]. However, BMN673's potential role in modulating parthanatos and attenuating cell death in COPD remains a subject for further investigation [121].

Research has demonstrated that multiple compounds confer anti-apoptotic effects by selectively modulating key pathways involved in apoptosis (Table 1). As previously noted, Bax and Bcl-2 are critical proteins of the Bcl-2 protein family, functioning as pro-apoptotic and anti-apoptotic factors, respectively, and jointly mediating the process of programmed cell death through the mitochondrial apoptosis pathway. Asiaticoside activates the PGC-1α/Nrf2 pathway, resulting in downregulation of Bax and upregulation of Bcl-2. This action not only mitigates inflammation but also suppresses EMT, thereby reducing small airway wall thickening in CS-induced lung tissue [122]. Another study showed that Korean Red Ginseng can also counteract the toxicity of cigarette smoke condensate (CSC), significantly reducing apoptosis levels in NCI-H292 cells and mouse lung tissue by inhibiting the overactivation of the VEGF/PI3K/AKT pathway, which alleviates alveolar damage and emphysema [123]. Caspase-3, the central executioner of apoptosis, integrates signals from multiple pathways and induces DNA fragmentation, leading to cell death. Formononetin (FMN) [124] and N-acetyl-L-cysteine (NAC) [125] were reported to inhibit the AhR/CYP1A1/AKT/mTOR pathway, while NAC also replenishes GSH and suppresses the p53 pathway. Both compounds significantly reduce Caspase-3 activation, alleviating alveolar damage and demonstrating therapeutic potential for restoring lung function.

Mounting evidence further highlights oxidative stress as a key regulator of apoptosis, making it an attractive therapeutic target [129]. Anthrahydroquinone-2,6-disulfonate (AH₂QDS) is a labdane diterpene lactone compound derived from Andrographis paniculata that exhibits anti-inflammatory, antioxidant, and antiviral activities. Li and colleagues [126] demonstrated that AH₂QDS activates the Keap1/Nrf2 pathway, upregulating downstream antioxidant proteins HO-1 and NQO1, which enhances cellular antioxidant capacity, reduces inflammation, and decreases apoptosis in rat lung tissue induced by LPS and CS, ultimately contributing to the restoration of lung function. Progesterone (P4), a key steroid hormone, was documented to also protect breast cancer cells and uterine leiomyoma tissue from oxidative stress-induced apoptosis [130]. Li et al. [128] further reported that P4 activates the c-MYC/SIRT1/PGC-1α pathway to maintain the function of the mitochondria and reduce H₂O₂-induced apoptosis in BEAS-2B cells and airway smooth muscle cells. This suggests that P4 may have therapeutic potential beyond reproductive health, particularly in ameliorating the symptoms and advancement of COPD. In addition, Gaylussacin, a potential therapeutic agent for COPD, markedly suppresses MMP-12 expression and activity in macrophages by activating the SIRT1 pathway, thereby mitigating pulmonary inflammation, oxidative stress, and apoptosis in models induced by lead/cadmium (Pb/Cd) or porcine pancreatic elastase (PPE). These effects lead to improved lung function and slowed disease progression. Additionally, its favorable oral bioavailability and safety profile further support its therapeutic promise [127].

Intervening in necroptosis-related key kinases has shown therapeutic potential in COPD (Table 2). GSK'547 is a RIPK1 inhibitor that significantly reduces elastase-induced emphysema and lung function decline, diminishes CS-induced peri-bronchial inflammatory cell infiltration, airway collagen deposition, and emphysema-like changes [52]. Necrostatin-1 (NEC-1), another RIPK1 inhibitor, has been reported to suppress cigarette smoke-induced inflammatory cytokine secretion from bone marrow–derived macrophages (BMDMs) [51]. However, Chen et al. [56] suggested that NEC-1 does not alleviate cigarette-induced emphysema and airway inflammation, and hypothesized that in COPD, there may exist mechanisms of RIPK3 activation that are independent of RIPK1. In contrast, RIPK3 inhibitor GSK'872 significantly reduces smoking-induced lung macrophage and epithelial cell death [51,56] and markedly improves COPD-related pathological changes [56]. The discrepancy in findings regarding NEC-1's effects on CSE-induced cell death may result from differing assessment methods. To further clarify RIPK1's role, gene knockout models are needed. Notably, Hannelore et al. [52] further employed single-cell sequencing and demonstrated elevated RIPK1 expression in multiple lung cell types in both COPD patients and experimental mouse models. Genetic or pharmacological inhibition of RIPK1 using GSK' 547 markedly lowered airway inflammation following acute and subacute exposure to CS, reduced apoptosis and necrosis induced by chronic exposure, and mitigated airway remodeling and emphysema. Collectively, these findings reveal that RIPK1 is upregulated in COPD and represents a potential therapeutic target, though its interaction with RIPK3 remains to be fully defined.

Additionally, traditional Chinese medicine extracts targeting necroptosis have also shown promising effects in COPD treatment. Theaflavin 3,3′ digallate (TF-3), an extract from black tea, significantly improves CS-induced lung inflammation and emphysema. TF-3 inhibits necroptosis by reducing RIPK3 and MLKL phosphorylation [131]. Thymoquinone (Tq), an active ingredient extracted from black cumin, inhibits RIPK1 and RIPK3 expression, reduces MLKL phosphorylation, and attenuates CSE-induced epithelial cell death. Its protective effects on epithelial cells warrant further investigation in preclinical studies [132].

Various chemical agents targeting ferroptosis reduce cell death and ameliorate COPD pathology [133,134,135] (Table 3). Ferrostatin-1 (Fer-1) is a ferroptosis inhibitor. Deferoxamine (DFO) chelates iron, reducing intracellular iron levels. Both Fer-1 and DFO markedly reduce CS-induced epithelial cell death, which in turn decreases inflammation and potentially helps mitigate inflammatory cell infiltration and lung injury in COPD [135,136,137,138]. In addition, Fer-1 inhibits macrophage M2 polarization induced by epithelial cells [139]. Plant extracts can also improve COPD pathology by regulating ferroptosis. Ginsenoside Rg1 [140], Ginkgo biloba extract (GBE) [141], and Sea buckthorn extract (SBE) [142] can effectively reduce inflammatory cell infiltration and promote alveolar repair by triggering the Nrf2/SLC7A11/GPX4 pathway and inhibiting the occurrence of ferroptosis. Acacetin [143], Curcumin (CUR) [136], scutellarein (STR) [36], and dihydroquercetin (DHQ) [144] are natural compounds that significantly inhibit CS-induced ferroptosis, thereby alleviating airway inflammation and ameliorating emphysema. CUR and DHQ further enhance cellular antioxidant capacity by promoting System Xc⁻ and glutathione (GSH) expression [36,136]. In addition to promoting GSH and NRF2, DHQ also inhibits ALOX15, reducing lipid peroxidation [144]. Other antioxidants, including hydrogen sulfide (H₂S) and sodium pyruvate (NaPyr), exert similar protective effects by upregulating NRF2, GSH, and GPX4, suppressing CS-induced epithelial ferroptosis and inflammation, and alleviating PM2.5-triggered airway injury [145]. Moreover, Lipoxin A4 (LXA4), an endogenous lipid mediator produced through lipoxygenase-dependent metabolism of arachidonic acid, exhibits both anti-inflammatory and antioxidant effects [146]. Li et al. [147] demonstrated that LXA4 activates the ALX/FPR2 receptor and inhibits p38/MAPK phosphorylation, suppressing ferroptosis and inflammatory responses while improving lung function and tissue integrity. However, clinical translation of LXA4 is hindered by its rapid inactivation and short half-life. Consequently, developing stabilized LXA4 analogs represents a potentially viable treatment approach for COPD.

Various agents can precisely modulate the balance between LC3B II and p62 expression, restoring autophagic homeostasis and alleviating lung injury (Table 4). The traditional Chinese medicine formula Bufei Yishen Formula (BYF) and its active components effectively upregulate LC3B II and downregulate p62, activating the autophagy pathway to promote the clearance of damaged organelles and suppress cellular senescence. It also mitigates inflammation and oxidative stress, preserves epithelial barrier function, and improves lung performance [148,149]. Similarly, natural compounds such as dihydromyricetin (DHM) [150] and quercetin [151], as well as the sildenafil analog vardenafil [152], can restore autophagy function, clear CS-induced lung tissue damage components, and improve lung function. DHM additionally reduces mucus hypersecretion and repairs ciliary dysfunction by rebalancing autophagy [150]. Conversely, when excessive autophagy contributes to cell injury, diindolylmethane (DIM) has demonstrated regulatory effects. Jung et al. [153] showed that in HEL299 human lung fibroblasts exposed to cadmium, a recognized risk factor for COPD, DIM suppresses excessive autophagy and attenuates autophagy-related cell death, revealing its potential as a viable treatment candidate for cadmium-associated diseases, including COPD.

Pharmacological regulation of mitophagy has also been recognized as a therapeutic avenue for COPD, either by repressing its overactivation or restoring its impaired activity. Puerarin, an isoflavone derived from kudzu root, exhibits antioxidant and anti-apoptotic properties. By activating the PI3K/AKT/mTOR pathway, it inhibits CSE-induced excessive mitophagy, as demonstrated by Wang et al. [154], thereby rescuing mitochondrial function and protecting bronchial epithelial cells from CSE-induced injury, providing a novel potential strategy for COPD therapy. In contrast, hydrogen sulfide (H₂S) has been reported to enhance impaired Parkin-mediated mitophagy in a PM2.5 model by upregulating Klotho expression and inhibiting IGF-1R, clearing abnormal mitochondria, and alleviating cellular senescence and inflammation [88]. These effects significantly mitigate emphysema and airway inflammation, inhibit COPD progression, and underscore the potential clinical applications of H₂S-based therapies. Collectively, these findings emphasize the importance of maintaining mitophagy at an appropriate level to preserve mitochondrial homeostasis and slow disease progression.

The strategy of suppressing pyroptosis by inhibiting NLRP3 inflammasome activation, blocking inflammatory caspase maturation, and preventing GSDMD cleavage offers a promising therapeutic approach for COPD (Table 5). A variety of natural plant extracts, including Resveratrol [155], DHM [156], Schisandrin A (SchA) [157], Tanshinone(TS) [158], grape seed proanthocyanidin extract(GSPE) [159], Daphnetin(Daph) [160], osthole [161], the flower buds of Tussilago farfara L (FTF) [162], (−)-Epicatechin(EC) [163], and isoforskolin (ISOF) [164], have demonstrated therapeutic efficacy in COPD treatment. These extracts inhibit lung cell pyroptosis by suppressing the NLRP3 inflammasome, alleviating airway inflammation and emphysema, and improving lung function. For instance, DHM can directly bind to and stabilize SLC7A11, enhancing glutathione synthesis and subsequently inhibiting lipid peroxidation and pyroptosis [156]. SchA also activates the NRF2/HO-1 signaling pathway, an important target in COPD treatment [157], whereas TS inhibits both NF-κB activation and the secretion of inflammatory cytokines IL-6 and IL-8 [158]. Daph additionally reduces airway inflammation and mucus hypersecretion resulting from PM2.5 exposure or cigarette smoke [160]. Consistent with these findings, traditional Chinese medicine exhibits complex pharmacological properties, particularly in regulating pyroptosis. Both Astragaloside IV [165] and Tianlong Kechuanling (TL) [166] were reported to control the NLRP3-GSDMD signaling pathway, thus participating in the regulation of pyroptosis. Notably, TL inhibits pyroptosis in pulmonary vascular endothelial cells, alleviates inflammatory responses and vascular remodeling, and consequently reduces pulmonary arterial hypertension while improving pulmonary function. It shows great clinical promise for COPD complicated by pulmonary arterial hypertension.

In addition, other chemical agents targeting pyroptosis have also shown potential in COPD therapy [168]. Hydrogen sulfide, a chemical agent, alleviates smoking-induced lung inflammation [169,171]. Magnesium isoglycyrrhizinate (MgIG), a derivative of glycyrrhizic acid, mitigates smoking and LPS-induced lung function decline, suppressing airway inflammation, remodeling, and emphysema [172]. A range of specific inhibitors has currently been developed primarily for use in experimental research settings, such as the NLRP3 inhibitor MCC950 [170] and the caspase-1 inhibitor VX-765 [105], both of which markedly attenuate lung inflammation. As previously noted, GSDMD is the central executor of pyroptosis, mediating lytic and inflammatory cell death. Gao et al. [167] demonstrated that disulfiram (DSF) directly targets GSDMD and, through covalent binding, prohibits the pore-forming activity of its N-terminal fragment (GSDMD-NT), thus preserving airway epithelial barrier integrity and attenuating ozone-induced COPD pathology and lung function impairment. Given its established safety profile as an approved drug, DSF holds significant translational potential as a therapeutic agent for COPD. However, its efficacy and mechanism of action in CS-induced models should be well-established in future research to support clinical application. Notably, although AIM2-mediated pyroptosis also participates in the development of COPD [96,173], little is currently known about pharmacological agents capable of regulating the AIM2 inflammasome.