The Role of Cellular Senescence in Chronic Lung Diseases: Emerging Mechanisms and Translational Perspectives: A Narrative Review

Abstract

Cellular senescence is one of the major risk factors for the onset and progression of chronic pulmonary diseases. Cellular senescence can be induced by diverse stressors, including genotoxic damage, oncogenic signaling, and therapeutic interventions. These senescent cells communicate via the release of multiple inflammatory molecules known as the Senescence-Associated Secretory Phenotype (SASP), which induces persistent low-grade inflammation and contributes to various chronic inflammatory lung diseases. This review summarizes the basic concepts of cell senescence, its hallmarks, SASP, and the mechanisms of cell senescence in the lung, and its consequences in the development and progression of chronic pulmonary diseases. Current therapeutic strategies include senolytics (e.g., BCL-2 family inhibitors and dasatinib–quercetin) and senomorphics that suppress SASP activity. Future directions in the development of cell- and stage-specific therapies are critical for targeting age-related lung disease with desired outcomes.

1. Introduction

Chronic respiratory diseases are a leading cause of global morbidity and mortality among non-communicable diseases, affecting both the airways and lower respiratory tract [1,2,3]. Chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), pulmonary arterial hypertension (PAH), asthma, pneumoconiosis, interstitial lung disease (ILD), and pulmonary sarcoidosis are considered chronic pulmonary diseases [1]. Despite the availability of therapies that improve quality of life, to control disease progression, attenuate exacerbations, and prevent other possible adverse outcomes and disability, there is no cure for chronic respiratory diseases [4]. The age-standardized mortality rates of chronic respiratory diseases decreased before the pandemic and increased thereafter; the age-standardized incidence rates showed a moderate rise during the pandemic [5]. Aging is a progressive biological process characterized by the accumulation of molecular and cellular damage [6,7]. The human lungs have the largest surface area among internal organs. They are exposed to the external environment throughout life with various stressors, such as chemical, mechanical, biological, and immunological stressors, which cause impairment of lung function characterized by impaired exchange of gases with advancing age [8]. Chronic lung diseases, although affecting a single organ, the lung, have diverse triggering factors, pathological features, and clinical presentations, yet share a similar mechanism of cellular senescence affecting different types of lung cells across diverse lung conditions. Cellular senescence is a state of cell cycle characterized by phenotypic changes that lead to pathological conditions in various age-related diseases [9]. López-Otín C et al., in 2013, suggested nine hallmarks of aging, namely, DNA instability, telomere attrition, epigenetic alterations, loss of proteostasis, dysregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication [10]. They recently included three additional hallmarks of aging, namely, disabled macroautophagy, chronic inflammation, and dysbiosis [11]. Cellular senescence is one of the major risk factors for the development of age-related chronic lung diseases. Although it plays an important role in maintaining physiological homeostasis, it also contributes to disease pathology. In this review, we will discuss the hallmarks of senescence, senescence in pulmonary cells, the mechanisms underlying their contribution to the development of chronic lung diseases, and the available therapies.

2. Cellular Senescence: Concepts and Hallmarks

2.1. Types of Cellular Senescence

Cellular senescence is divided into four types: (a) replicative senescence, (b) stress-induced premature senescence, (c) oncogene-induced senescence, and (d) therapy-induced senescence. Replicative senescence is primarily driven by telomere shortening and genomic instability resulting from repeated cell division [12,13]. Exposure to physical or chemical stress induces oxidative stress and DNA damage, transforming normal cells into senescent cells, a process known as Stress-Induced Premature Senescence [14]. Oncogenic activation is considered a tumor-promoting event when it promotes cell proliferation; it can also induce stress, known as oncogenic stress, and induce irreversible cell cycle arrest in cultured cells or tumor tissues [15]. For example, the cultured human primary lung fibroblast line IMR90 undergoes cellular senescence in the presence of a Ras oncogenic mutation [16]; whereas a similar behavior is observed in mammary epithelial cells in vivo, where Ras is overexpressed [17]. This type of senescence is known as oncogene-induced senescence (OIS). On the other hand, therapy-induced senescence is observed when the cells undergo cell cycle arrest after exposure to radiation or certain chemotherapies [18,19] through the induction of DNA double-strand breaks (DSBs) [20].

2.2. Hallmarks and Biomarkers of Senescence

The cell cycle is an essential process for both development and viability of multicellular organisms. However, cell cycle arrest implies the cessation of the cell cycle, and it is one of the markers of senescent cells. The cell cycle arrest might be a protective pathway in response to harmful stimuli to prevent the propagation of dysfunctional cells (Figure 1) [21]. Activation of the p53/p21 and/or p16-pRB tumor suppressor pathways mediates cell cycle arrest during senescence. Sustained overexpression of any of the four critical components (p53, pRB, p16^INK4A, p21^WAF1/CIP1) is sufficient to trigger cellular senescence [22,23,24,25]. In the p53/p21^WAF1/CIP1 pathway, p53, known as ‘Guardian of the genome,’ is activated by multiple post-translational modifications, namely phosphorylation, methylation, acetylation, sumoylation, ubiquitination, and neddylation, which leads to the expression of various antiproliferative genes and also activates p21^WAF1/CIP [26,27,28,29]. p21^WAF1/CIP1, a 21 kDa protein, is a cyclin-dependent kinase inhibitor of the Cip/Kip family, which inhibits cell proliferation [30]. It further inhibits the phosphorylation of the pRB family of proteins and pRB/E2F complex formation, leading to cell cycle arrest [31,32]. On the other hand, in the p16^INK4A/pRB pathway, p16^INK4A is a 16 kDa protein that suppresses the formation of cyclin D-CDK4/6 complex by binding to CDK4/6, hence preventing phosphorylation of pRB and forming a pRB-E2F repressive complex and inhibiting the transcription of genes required for cell proliferation [33,34].

Nuclear DNA is the most vital component of a cell, where any kind of DNA damage activates the DNA damage response (DDR) by suppressing cell proliferation to avoid its expression in daughter cells and activating the DNA damage repair process to maintain its integrity [35]. DNA damage can result in single-strand lesions or double-strand breaks (DSBs), both of which activate the DNA damage response (DDR). At the site of a DNA lesion, these further activate two large protein kinases, namely, ataxia telangiectasia and Rad3-related (ATR) or ataxia telangiectasia mutated (ATM), respectively, which phosphorylate the H2AX histone, which is an essential step in nucleation of DDR [35,36,37]. The DDR mediators, mediator of DNA damage checkpoint 1 [38,39,40] and p53 binding proteins [41], are important for establishing this positive feedback loop. An increase in ATM and ATR activity beyond the threshold activates checkpoint kinase CHK2 and checkpoint kinase CHK1 upon phosphorylation of ATM and ATR, respectively, which further freely diffuses throughout the nucleoplasm [42,43,44]. In response to DNA damage, checkpoint activation triggers various signaling pathways. It inactivates cell division cycle 25 (CDC25) phosphatases, which leads to cell cycle arrest, as these phosphatases are essential for cell proliferation. However, on the other hand, a delayed activation of p53 by phosphorylation of DDR kinases induced p21 expression, further contributing to cell-cycle arrest [45,46,47]. Moreover, the DNA damage response (DDR) is also activated by telomere dysfunction. Telomeres are the regions at the end of linear chromosomes that are protected by a set of proteins to prevent the DNA damage response. Telomerases, a DNA template-independent DNA polymerase, replicate these telomeres. However, in the absence of telomerase, telomeres progressively shorten, leaving their ends uncapped and unprotected [48,49]. This results in exposure of DNA ends and triggers DNA damage response pathways, which further activate p16 and p21, ultimately leading to cell cycle arrest [50,51].

• Mitochondrial dysfunction and metabolic reprogramming

Decreased respiratory capacity per mitochondrion, along with a decrease in mitochondrial membrane potential (ΔΨm) at steady state in senescent cells, is considered the defining feature of mitochondrial dysfunction. In senescent cells in vitro, the mitochondrial mass is increased due to decreased mitophagy, and accumulation of dysfunctional mitochondria might act as a compensatory mechanism for decreased mitochondrial function. [52,53] Dysfunctional mitochondria with reduced ΔΨm contribute to increased production of reactive oxygen species (ROS) [54,55]. The mitochondrial dysfunction in senescent cells is associated with immunosenescence and metabolic reprogramming [56]. The glucose metabolism is reprogrammed in senescent cells, where there is a significant metabolic shift towards glycolysis over oxidative phosphorylation (OXPHOS), known as a Warburg-like metabolic shift. This shift predominantly increases reactive oxygen species (ROS) production due to mitochondrial dysfunction [57]. It also redirects glucose-derived carbon toward lipid biosynthesis and thereby promotes age-related lipid droplet accumulation [58]. Age-related lipid accumulation in senescent cells activates the PI3K/AKT/mTOR pathway, thereby activating de novo fatty acid synthesis via acetyl-CoA synthase and fatty acid synthase while inhibiting mitochondrial β-oxidation, ultimately leading to the formation of lipid peroxidation products such as 4-HNE, which damage the mitochondrial membrane and impair oxidative phosphorylation [57,58,59]. These lipid droplets inhibit mitophagy via p62-dependent mechanisms [58]. This process is known as lipid metabolic reprogramming. NF-kB signaling is activated when the balance between glycolysis and OXPHOS is disrupted, thereby enhancing the SASP [56]. Glutamine metabolism is considered a major adaptive mechanism in aging cells by maintaining NADPH production through the malic enzyme 1 (ME-1) pathway [60]. However, during metabolic reprogramming, the mTOR pathway is impaired by dysregulated glutamine metabolism, thereby exacerbating mitochondrial dysfunction and leading to cellular senescence [56,61].

• Epigenetic alterations

The word “epigenetics” is derived from the Greek word “epi” and means “over” or “above” the genome. Epigenetics represents a reversible mechanism for regulating the function of the genome without altering its underlying DNA sequence; thus, the epigenome links genotype to phenotype and plays an important role in modulating the aging process in response to environmental stimuli [62]. Epigenetic mechanisms critically regulate aging trajectories through dynamic DNA methylation, histone modifications, chromatin remodeling, and non-coding RNA (ncRNA)-mediated pathways, directly influencing age-related disease pathogenesis [60].

a.

DNA methylation:

The key epigenetic modification in mammals is methylation at the 5th position of cytosine in DNA, which results in the formation of 5-methylcytosine, which suppresses gene expression by recruiting proteins or by preventing transcription factor binding to the gene [63]. The actions of the ten-eleven translocation protein family on 5-methylcytosine lead to the production of a stable modified DNA base known as 5-Hydroxymethylcytosine (5hmC) [64]. Cytosine modifications are low in senescent cells compared with proliferating or quiescent cells [64], characterized by a decrease in DNA methylation predominantly in heterochromatin regions and hypermethylation in the regions of promoter CPGs, which is known as “epigenetic drift” [65,66]. This correlation between age and DNA methylation is described as the epigenetic clock by Steve Horvath [67].

b.

Histone modification:

There are several types of histone modification, namely: acetylation, methylation, phosphorylation, glycosylation, ADP-ribosylation, deamination, ubiquitination, and proline isomerization, where acetylation and methylation are mainly associated with senescence. The methylation levels are altered in the presence of histone methyltransferase or histone demethylases, where these alterations participate in transcriptional activation or transcriptional repression via methylation at lysine 4 of histone (H3K4), H3K36, H3K79, H3K27, and H4K20, respectively [68], which is concluded to be altered in senescence [69]. Histone acetyltransferases and histone deacetylases play a major role in longevity, which are transcriptional activators and transcriptional repressors, respectively [69].

c.

Chromatin remodeling:

Aging causes the loss of histones and remodeling of chromatin, where the epigenomes of senescent cells exhibit characteristics such as loss of chromatin rigidity, increased entropy, reduced compartmentalization, disorganization of epigenomes, convergent changes in genome-wide epigenetic signatures, and reduced polarity [70,71,72].

d.

Transcriptional alterations:

Short non-coding RNA (snRNA), small interfering RNA (siRNA), microRNA (miRNA), circular RNA (circRNA), PIWI-interfering RNA (piRNA), endogenous siRNA (endo-siRNA), and long ncRNA (lncRNA) are considered single-stranded RNAs that are found to be produced during the aging process from an unstable genome along with dsRNA [73,74].

• Microbiota dysbiosis and cellular senescence

Recently, there has been increasing interest in the association between cellular senescence and the microbiome and its contribution to the aging process. A study by Kawamoto et al. revealed an unexpected interplay between gut microbiota and B cell senescence via IgA, which gives us a new perspective on cell senescence and gut microbiota with aging [75]. Microbiome dysbiosis triggers oxidative stress and the production of bacterial short-chain fatty acids, which promote senescence [76,77]. The lung microbiome contributes to maintaining lung homeostasis and is altered in chronic lung diseases such as COPD and asthma [78]. There is an increased risk of exacerbations and inflammation, and it may also induce lung senescence via elevated oxidative stress due to increased neutrophil recruitment. The lung microbiome and oral and intestinal microbiota are interdependent. However, the role of lung microbiota dysbiosis in lung senescence is not yet established.

• Senescence-associated β-galactosidase staining

In senescent cells, increased lysosomal degradation results in elevated β-galactosidase activity, which serves as a primary basis of SA-β-gal staining (Senescence-associated β-gal staining) [79]. The strong SA-β-Gal activity is observed in senescent cells, where the β-Galactosidase activity is detected at pH 6.0, which hydrolyzes the soluble chromogenic substrate 5-bromo-4-chloro-3-indoyl β-D-galactopyranoside into an insoluble product (Blue). The GLB1 gene encodes lysosomal β-galactosidase, where its mRNA and protein levels are increased in senescent cells, contributing to increased β-galactosidase activity [79,80,81]. In doxorubicin-induced senescence, SPiDER-SA-Beta-gal has been used, where Hoechst stain was used to counterstain the nucleus [82,83]. Although SA-β-gal is widely used to detect senescent cells, paraffin-embedded tissue sections and live cells are not an ideal choice for this assay. Moreover, the method lacks reliability and specificity [84,85].

2.3. SASP

The cumulative DNA damage response, cell cycle arrest, and accumulation of defective organelles, such as mitochondria and lysosomes, lead to SASP [86,87], which is composed of proteins, proteases, growth factors, cytokines, and chemokines. The SASP is composed of proteins that include interleukins, cytokines, chemokines, growth factors, and ECM proteases (Table 1) [88]. Among interleukins and cytokines, IL-1α and IL-1β are the most studied molecules, which further activate IL-6 and IL-8, which induce paracrine senescence, cell cycle arrest, and immune response [89,90]. IL-1α also activates the transcription of pro-inflammatory SASP, only when caspase 6-cleaved IL-1α is translocated from the cell surface to the nucleus [91,92]. Moreover, IL-1α and IL-1β also regulate a strong chemoattractant SASP, the CCL2 chemokine, which regulates monocyte and macrophage migration and infiltration, whereas CCL5 is secreted by TIS cancer cells [93]. CXCL chemokines such as CXCL1, CXCL2, and CXCL3 are secreted by stellate senescent cells in the liver and pancreas to perform paracrine function by binding to C-X-C chemokine receptor type 2 (CXCR2) [94]. Transforming growth factor β is a SASP factor and upregulates SASP by inducing paracrine senescence in a hypoxic tumor environment. On the other hand, HGF may antagonize tumor-suppressive mechanisms, which is mainly secreted by senescent fibroblasts [95]. Other than cytokines, growth factors and matrix metalloproteinases (MMPs) are the proteases secreted as a part of SASP. MMP-1 (collagenase-1), MMP-3, and MMP-10 are majorly upregulated during replicative or stress-induced senescence in mouse fibroblasts (Table 2) [96,97].

• Temporal dynamics of SASP

The induction of SASP is highly regulated by various pathways, such as the p53/p21 pathway, the p16-pRB pathway, the DNA damage response (DDR) pathway, the NF-kB pathway, the p38 MAPK and mTOR pathway, and the cytosolic DNA-cGAS-STING pathway. However, pathway regulation varies with the phase of senescence, such as the p53/p21 and NF-kB pathways, which are early and late regulators of senescence, whereas LINE1-driven interferon signaling is considered a marker of deep senescence [98]. On the other hand, the SASP factors secreted also define the stage of senescence, namely TGF-β in the early phase and IL-6 and IL-8 in the late phase. Moreover, transcription factor activator protein 1 (AP1) was considered to define the temporal dynamics of SASP from a multiple-omics study [99].

Table 1. Senescence-specific SASP factors.

SASP Factors	Replicative Senescence	DNA Damage-Induced Senescence	Oncogene-Induced Senescence	Therapy-Induced Senescence
Interleukins, cytokines, and their receptors
IL-1α, IL-1β, IL-6	+	+	+	+
IL-1R1, IL-1R2-α	-	+	-	-
IFN-γ, LIF	-	-	+	-
IL-11, IL-15	+	+	-	-
IL-6R, Oncostatin M	-	+	+	-
IFN-1	+	-	+	-
Chemokines and their receptors
CTACK, RANTES	-	+	-	-
CCL1, CCL2, CCL7, CCL20, CXCL-5, CXCL-6	-	-	+	-
CXCL8	-	-	-	+
MIP-1α, MIP-3α, MIF, MCP-1, 2, 4, Eotaxin-3, GROα,β,γ	+	+	+	-
ENA-78, GCP-2, I-309, I-TAC,	-	+	+	-
CXCR2	+	-	+	-
Proteases
MMP-3	+	+	+	+
MMP-12, MMP-13, MMP-14	-	+	-	-
MMP-1, MMP-10	+	+	+	-
MMP-2	+	+	-	+
Growth factors and their receptors
TGF-β	+	-	-	-
BTC, MSP-a, PDGF-BB	-	+	-	-
IGFBP7, G-CSF	-	-	+	-
IGFBP1, 2,3,5, FGF-7	+	+	-	-
SDF-1, VEGF	-	+	+	-
AREG	-	-	+	+
bFGF, GM-CSF, HGF, IGFBP-4, 6, PIGF	+	+	+	-
Others
Fibronectin	+	-	-	-
TIMP-2	+	+	+	-
COX-2, PGE-2	+	-	+	-
tPA	+	+	-	+

AREG: Amphiregulin, COX-2: Cyclooxygenase-2, CXCR: CXC chemokine receptor, bFGF: basic fibroblast growth factor, EGF: endothelial growth factor, GMCSF: granulocyte–macrophage colony stimulating factor, GRO: growth-related oncogene, HGF: hepatocyte growth factor, IFN: interferon, IGFBP: insulin-like growth factor binding protein, IL: interleukin, MCP: membrane cofactor protein, MMP: matrix metalloproteinases, PDGF: platelet-derived growth factor, PGE2: prostaglandin E2, PIGF: placental growth factor, SDF: stromal cell-derived factor, TGF-β: transforming factor-β, VEGF: vascular endothelial growth factor, MIP: macrophage inflammatory protein, MIF: macrophage migration inhibitory factor, GCP: granulocyte chemotactic protein, TIMP: tissue inhibitor of metalloproteinases. The data is summarized from Kuilman et al. [100], Coppe et al. [101], Freund et al. [102], Lopes et al. [103].

Table 1. Senescence-specific SASP factors.

SASP Factors	Replicative Senescence	DNA Damage-Induced Senescence	Oncogene-Induced Senescence	Therapy-Induced Senescence
Interleukins, cytokines, and their receptors
IL-1α, IL-1β, IL-6	+	+	+	+
IL-1R1, IL-1R2-α	-	+	-	-
IFN-γ, LIF	-	-	+	-
IL-11, IL-15	+	+	-	-
IL-6R, Oncostatin M	-	+	+	-
IFN-1	+	-	+	-
Chemokines and their receptors
CTACK, RANTES	-	+	-	-
CCL1, CCL2, CCL7, CCL20, CXCL-5, CXCL-6	-	-	+	-
CXCL8	-	-	-	+
MIP-1α, MIP-3α, MIF, MCP-1, 2, 4, Eotaxin-3, GROα,β,γ	+	+	+	-
ENA-78, GCP-2, I-309, I-TAC,	-	+	+	-
CXCR2	+	-	+	-
Proteases
MMP-3	+	+	+	+
MMP-12, MMP-13, MMP-14	-	+	-	-
MMP-1, MMP-10	+	+	+	-
MMP-2	+	+	-	+
Growth factors and their receptors
TGF-β	+	-	-	-
BTC, MSP-a, PDGF-BB	-	+	-	-
IGFBP7, G-CSF	-	-	+	-
IGFBP1, 2,3,5, FGF-7	+	+	-	-
SDF-1, VEGF	-	+	+	-
AREG	-	-	+	+
bFGF, GM-CSF, HGF, IGFBP-4, 6, PIGF	+	+	+	-
Others
Fibronectin	+	-	-	-
TIMP-2	+	+	+	-
COX-2, PGE-2	+	-	+	-
tPA	+	+	-	+

AREG: Amphiregulin, COX-2: Cyclooxygenase-2, CXCR: CXC chemokine receptor, bFGF: basic fibroblast growth factor, EGF: endothelial growth factor, GMCSF: granulocyte–macrophage colony stimulating factor, GRO: growth-related oncogene, HGF: hepatocyte growth factor, IFN: interferon, IGFBP: insulin-like growth factor binding protein, IL: interleukin, MCP: membrane cofactor protein, MMP: matrix metalloproteinases, PDGF: platelet-derived growth factor, PGE2: prostaglandin E2, PIGF: placental growth factor, SDF: stromal cell-derived factor, TGF-β: transforming factor-β, VEGF: vascular endothelial growth factor, MIP: macrophage inflammatory protein, MIF: macrophage migration inhibitory factor, GCP: granulocyte chemotactic protein, TIMP: tissue inhibitor of metalloproteinases. The data is summarized from Kuilman et al. [100], Coppe et al. [101], Freund et al. [102], Lopes et al. [103].

Table 2. Representative SASP-associated mediators reported in senescent lung cell populations.

Disease	Type of Cell	SASP Factor	Reference
Chronic Obstructive Pulmonary Disease (COPD)	Bronchiolar epithelial cells	IL-1α, IL-1β, IL-6, IL-8, IL-13, IL-8, GRO-α, GRO-β, GRO γ, MCP-1	[104,105,106,107]
	Alveolar macrophages	MMP-9, MMP-1	[108,109]
	Fibroblasts	ICAM-1, ICAM-3, osteoprotegerin, TRAIL-R3, sTNFR1, Fas, sTNFR2, uPAR, and endothelial growth factor-R	[110]
Pulmonary Fibrosis	Fibroblasts	TNF-α, TGF-β, IL-1β, IL-6, IL-8, IL-10, IL-18, CXCL1, MCP1, FGF, CTGF, GM-CSF, M-CSF, PDGF, LTA4, LTB4, LTC4, LTD4	[111,112,113,114,115,116,117,118]
Pulmonary Arterial Hypertension	Smooth muscle cells	IL-6, Osteopontin	[119,120]
Asthma	Immune cells	IL-6 and TGF-β	[121]
	Bronchial fibroblasts	GM-CSF, TNF-α, IL-1β, and IL-6

GRO: growth-related oncogene, MCP: membrane cofactor protein, MMP: matrix metalloproteinases, ICAM: Intercellular Adhesion Molecule, uPAR: urokinase-type plasminogen activator receptor, TRAIL-R3: Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand Receptor-3, Stnfr-Soluble Tumor Necrosis Factor Receptor, TNF-α: Tumor Necrosis Factor, TGF-β: transforming growth factor-β, FGF: fibroblast growth factor, CTGF: connective tissue growth factor, GMCSF: granulocyte–macrophage colony stimulating factor, M-CSF: macrophage colony stimulating factor, PDGF: platelet-derived growth factor, LT: leukotrienes. The listed mediators represent SASP-associated factors reported in the indicated experimental or disease contexts and should not be interpreted as exclusive cell-type-specific SASP signatures. Many factors can originate from multiple senescent and non-senescent cell populations, particularly in complex tissue or biofluid samples.

Table 2. Representative SASP-associated mediators reported in senescent lung cell populations.

Disease	Type of Cell	SASP Factor	Reference
Chronic Obstructive Pulmonary Disease (COPD)	Bronchiolar epithelial cells	IL-1α, IL-1β, IL-6, IL-8, IL-13, IL-8, GRO-α, GRO-β, GRO γ, MCP-1	[104,105,106,107]
	Alveolar macrophages	MMP-9, MMP-1	[108,109]
	Fibroblasts	ICAM-1, ICAM-3, osteoprotegerin, TRAIL-R3, sTNFR1, Fas, sTNFR2, uPAR, and endothelial growth factor-R	[110]
Pulmonary Fibrosis	Fibroblasts	TNF-α, TGF-β, IL-1β, IL-6, IL-8, IL-10, IL-18, CXCL1, MCP1, FGF, CTGF, GM-CSF, M-CSF, PDGF, LTA4, LTB4, LTC4, LTD4	[111,112,113,114,115,116,117,118]
Pulmonary Arterial Hypertension	Smooth muscle cells	IL-6, Osteopontin	[119,120]
Asthma	Immune cells	IL-6 and TGF-β	[121]
	Bronchial fibroblasts	GM-CSF, TNF-α, IL-1β, and IL-6

GRO: growth-related oncogene, MCP: membrane cofactor protein, MMP: matrix metalloproteinases, ICAM: Intercellular Adhesion Molecule, uPAR: urokinase-type plasminogen activator receptor, TRAIL-R3: Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand Receptor-3, Stnfr-Soluble Tumor Necrosis Factor Receptor, TNF-α: Tumor Necrosis Factor, TGF-β: transforming growth factor-β, FGF: fibroblast growth factor, CTGF: connective tissue growth factor, GMCSF: granulocyte–macrophage colony stimulating factor, M-CSF: macrophage colony stimulating factor, PDGF: platelet-derived growth factor, LT: leukotrienes. The listed mediators represent SASP-associated factors reported in the indicated experimental or disease contexts and should not be interpreted as exclusive cell-type-specific SASP signatures. Many factors can originate from multiple senescent and non-senescent cell populations, particularly in complex tissue or biofluid samples.

2.4. Cellular Heterogeneity and Limitations in Defining the Origin of the SASP

SASP composition is highly heterogeneous and is determined not only by the senescent cell type but also by the senescence-inducing stimulus. Proteomic profiling has demonstrated that different senescent cells and senescence-inducing conditions generate secretomes that consist of hundreds of largely distinct proteins, with only a smaller subset of shared SASP components [122]. This concept is particularly important to chronic lung diseases. Fibrotic alveolar type II epithelial cells exhibit increased expression or secretion of multiple inflammatory and matrix-remodeling SASP-associated mediators, including MMP12, SERPINE1, SPP1, and WISP1, supporting a secretory phenotype capable of promoting epithelial dysfunction and profibrotic tissue remodeling [123]. Furthermore, primary alveolar epithelial cells obtained from IPF patients show an increase in senescence-associated cytokines, including IL-6 and IFN-β [124]. In comparison, senescent pulmonary artery smooth muscle cells (PASMC) exhibit a secretory phenotype with prominent pro-proliferative activity: paracrine IL-6 released by senescent PASMCs stimulates neighboring PASMC proliferation in experimental pulmonary hypertension, whereas osteopontin released by senescent PASMCs promotes vascular smooth-muscle proliferation and migration and is associated with p16-positive pulmonary vascular cells in pulmonary hypertension [119,120]. Thus, although some SASP components overlap across senescent cell populations, their relative abundance and biological consequences may differ substantially depending on cellular and disease context.

An important limitation is that the cellular origin of SASP mediators cannot be definitively determined from measurements obtained from whole-lung tissue, BALF, or other heterogeneous samples. Molecules that are commonly classified as SASP factors, including inflammatory cytokines, chemokines, growth factors, and matrix-remodeling proteins, are also produced by activated macrophages and other immune, epithelial, endothelial, fibroblast, and smooth-muscle populations independently of cellular senescence. Moreover, currently there is no single marker that can reliably identify senescent cells in vivo; therefore, a combination of senescence and cell-specific markers is generally required [123,125]. Although the single-cell sequencing technique can identify senescence-enriched epithelial populations in IPF, transcriptional detection of a candidate SASP factor does not necessarily demonstrate that the corresponding protein is secreted by the cell [126]. Future studies combining single-cell and spatial analyses with protein and secretome profiling will be needed to identify better cellular sources and functions of SASP factors in different chronic lung diseases.

3. Senescence in the Lung: Cell Type-Specific Perspectives

Human lung-resident cells are highly exposed to external environmental stressors such as pollutants, allergens, cigarette smoke, and microorganisms, compared to other mucosal surfaces, for a longer duration of time due to its unique interface between the outside environment and the internal host environment, leading to consistent cycles of injury and repair resulting in lung aging.

3.1. Airway and Alveolar Epithelial Cells

The airway epithelium maintains lung homeostasis by acting as a physical barrier between the internal and external environments and functioning as a defense system against environmental stressors and pathogens [127]. Physiological lung aging is characterized by high levels of reactive oxygen species, telomere shortening, inflammation, and increased senescence [9]. The expression of senescence markers is expected to be elevated in the airway epithelium of COPD patients and smoke-induced inflammatory mouse models [128]. Chronic exposure to cigarette smoke triggers a chronic immune response in epithelial cells, leading to emphysema, where alveolar epithelial cells are greatly affected, and peri-bronchial fibrosis results from airway remodeling (Figure 2) [129,130]. The alveolar epithelium gets injured upon exposure to various pollutants, toxins such as SO₂, O₃, NO_2, and cigarette smoke, which subsequently initiate repair responses to maintain lung homeostasis. However, persistent exposure and repair responses eventually impair the regenerative capacity of the epithelium, leading to emphysema and lung fibrosis [131,132]. Repetitive cell proliferation during repair processes shortens telomere length, thereby causing replicative senescence. Moreover, exposure to toxins induces oxidative stress and DNA damage, which may cause stress-induced senescence. Once epithelial cells become senescent, they enter the stage of cell cycle arrest and can no longer effectively perform repair functions, further promoting the progression of lung diseases. Cigarette smoking impairs the epithelial barrier function in vitro, where OCLN and ZO-1 junctional expression is disrupted [133,134,135]. Oldenburger A et al. concluded that E-cadherin mediated barrier function in airway epithelial cells is disrupted via downregulation of A-kinase anchoring protein (AKAP)-9 expression [136].

3.2. Lung Fibroblasts and Myofibroblasts

The accumulation of senescent cells, elevated chronic inflammation, and modified ECM are interlinked to lung aging and are considered a major aspect of pulmonary fibrosis [137]. In pulmonary fibrosis, senescent cells can either induce remodeling of the ECM or influence neighboring cells through SASPs [138,139]. These SASPs induce matrix remodeling components such as metalloproteinases (MMPs), collagen cross-linkers, and pro-inflammatory regulators, which affect the surrounding cells [140,141]. Moreover, a chronic inflammatory state promotes overexpression of ECM-degrading enzymes, chemokines, cytokines, and growth factors, resulting in a pro-fibrotic state [142]. Among these factors, TGF-β is considered a profibrotic cytokine that activates various signaling pathways, inducing profibrotic ECM remodeling, altering tissue structure, and ultimately contributing to reduced pulmonary function in both mice and humans [143]. A study by Andrew M. Hower et al. concluded that inhibition of TGF-β signaling during senescence associated with ECM remodeling led to improved collagen fiber organization [144]. In addition, the stiffening of ECM modulates myocardin-related transcription factor-A (MRTF-A) signaling, leading to the acquisition of a myofibroblast phenotype with expression of α -smooth muscle actin (α-SMA), which is responsible for fibrotic remodeling [145,146]. These myofibroblasts produce a large amount of ECM, especially collagen type I and fibronectin. However, these fibroblasts undergo apoptosis in the presence of the cytokine Fibroblast Growth Factor 2 (FGF2), which has been shown to exhibit antifibrotic activity in humans. It acts by antagonizing pro-fibrotic TGF-β signaling, inhibiting fibroblast activation, and preventing the transdifferentiation of non-fibroblasts to myofibroblasts [147]. The remodeling of the lung tissue matrix with aging plays a vital role in the development of idiopathic pulmonary fibrosis (IPF). Distinct morphological changes are observed in fibroblasts in IPF lung, such as reduction in cell division, increased cell size, increased senescence-associated β-galactosidase activity, and flattened morphology [139]. The induction of senescence in lung fibroblasts in IPF is driven by multiple mechanisms, including metabolic reprogramming, reduced apoptosis, oxidative stress, and mitochondrial dysfunction [148].

3.3. Endothelial Cells

The endothelium is the innermost layer of blood vessels, which participates in various physiologic functions such as homeostasis, substrate transfer, wound healing, and vascular tone [149]. Maintenance of endothelial integrity depends on regenerative capacity; however, its capacity is significantly impaired with aging due to the development of endothelial aging [150,151]. Endothelial senescence is triggered by various stressors, such as increased oxidative stress and reduced production of nitric oxide (NO), which are the key features of endothelial senescence. An age-related impairment in the production of endothelial nitric oxide synthase (eNOS) and its cofactors, tetrahydrobiopterin (BH4) and L-arginine, leads to reduced NO production and endothelial dysfunction [152]. Cell cycle arrest is promoted in a p53/p21-dependent manner during endothelial senescence via activation of protein kinase B (AKT), leading to inhibition of the transcription factor FOXO3a [153]. In endothelial senescence, the renin/angiotensin system (RAS) is altered, which can be identified by elevated angiotensin-Converting enzyme 1(ACE1), angiotensin II (Ang II), and angiotensin II type-1 receptor (AT-1). It is considered another essential component of vascular tone regulation [154]. Endothelial senescence leads to decreased endothelial nitric oxide production, impairing vasodilatation and indicating endothelial dysfunction, a marker of microvascular aging. Microvascular aging is considered a hallmark of biological aging [155]. Among human lung cells, pulmonary microvascular endothelial cells (PMECs) account for around 23% and are also considered a key component of the pulmonary microvasculature. The pulmonary blood-gas barrier is maintained by PMECs, where damage to these cells impairs the barrier function and contributes to increased pulmonary vascular resistance and hypertension in the case of dysfunctional PMECs, where PMECs are considered to be vital for pulmonary vascular remodeling [156]. Pulmonary hypertension is a rare progressive pulmonary vascular disease characterized by narrowing and obliteration of the distal lung arterial bed, elevated pulmonary arterial pressure, and resistance as a result of arterial remodeling. The senescent endothelial cells trigger the increased secretion of inflammatory cytokines and reactive oxygen species, which further disrupt endothelial function via endothelial SASP factors such as pro-inflammatory transcription factor nuclear factor-kB (NF-kB) [157,158]. A pro-inflammatory cocktail is released from senescent endothelial cells via SASP, namely IL-6, IL-1α, IL-1β, TNF-α, chemokines such as IL-8 and MCP-1/CCL2, and growth factor 2, which cause chronic low-grade inflammation in blood vessel walls [159]. Adhesion molecules such as VCAM-1 and ICAM-1 are upregulated in sEC, where circulating monocytes attach to these molecules, which are actively recruited by chemokines such as MCP-1, whereas CXCL8 attracts neutrophils, which trigger the release of Neutrophil Extracellular Traps (NETs), which causes initiation of low and chronic inflammation despite the removal of the initial trigger [160,161].

3.4. Immune Cell Senescence

In immunosenescence, the physiological structure and function of the immune system are deranged, leading to chronic low-grade inflammation known as “inflammaging” due to the accumulation of memory and non-functional immune cells, a pro-inflammatory environment, and a dysregulated immune system in which macrophages are considered mediators of immunosenescence [162]. Innate and adaptive immunity are regulated by macrophages, which protect against pathogens through phagocytosis [163]. The trigger of macrophage senescence can be due to gene mutation of Kras or exposure to lipopolysaccharides [164,165,166]. The physiologic function of macrophages is compromised during macrophage senescence, including reduced phagocytosis, altered autophagy, impaired metabolism, and accumulation of abnormal cells; however, chronic low-grade inflammation is observed in senescent macrophages, which is specific to tissues [167,168,169]. The SASP from macrophages alters nicotinamide adenine dinucleotide (NAD) metabolism due to the high expression of CD38, NADase [170,171]. CD38-positive macrophages accumulate in adipose tissue, altering lipolysis and NAD metabolism and impairing thermogenesis; this metabolic dysfunction is highly dependent on the NLRP3 inflammasome [172]. The liver-specific macrophages are known as Kupffer cells, which are important for liver homeostasis, similar to adipose tissue macrophages. CD83 accumulates during senescence, leading to elevated p21 CIP1 expression, an elevated signature of pro-inflammatory M1-like cells, and SIRT3-dependent mitochondrial dysfunction [173,174]. In immunosenescence, along with macrophages, neutrophils, and lymphocytes, neutrophils undergo age-related remodeling, yet neutrophil development and counts remain unaffected [175,176]. The phagocytic activity, chemotaxis, and superoxide generation capacity are impaired in old neutrophils from mice and humans, which may be due to enhanced or defective phosphoinositide 3-kinase (PI3K) signaling resulting from reduced expression of suppressor of cytokine signaling 1 (SOCS1) and SOCS3 [177,178,179,180]. The accumulation of senescent neutrophils leads to elevated IL-1β expression in the bone marrow of aged mice, thereby enhancing the skewing of hematopoietic stem cells toward myeloid differentiation [181]. The expression of intercellular adhesion molecule 1 (ICAM-1) was increased in pulmonary endothelial cells in aging lungs, resulting in excessive neutrophil-mediated inflammation and neutrophil retention, which contributed to lung damage [182,183]. On the other hand, T cell dysfunction is observed in T cell senescence [184]. The loss of CD27 and CD28 costimulatory molecules is observed in senescent T cells, leading to impaired T cell priming and TCR signal transduction [185,186,187]. These senescent immune cells release proinflammatory cytokines, chemokines, and proteases, which promote chronic low systemic inflammation.

4. Mechanism Driving Senescence in Chronic Lung Disease

4.1. Oxidative Stress and Mitochondrial Dysfunction

Smoking is considered one of the major risk factors for various diseases, but more significantly, pulmonary diseases. Smoking catalyzes the production of reactive oxygen species, which contribute to the pathogenesis of smoking-associated diseases [188]. Oxidative stress is typically due to an imbalance between oxidative and antioxidative systems in the given environment, which can result from excessive production of reactive oxygen species or decreased antioxidant capacity, such as superoxide dismutase (SOD), catalase, and glutathione (GSH) [189,190]. Cigarette smoke contains toxic chemicals that can cause oxidative stress and release reactive oxygen species and radicals, which damage the epithelial and connective tissue in smokers [191,192,193,194,195]. Epithelial injury promotes the release of proteolytic enzymes and reactive oxygen species (ROS), further exacerbating tissue damage [196]. The structural and functional abnormality due to ROS is the hallmark of CS-induced senescence in the lung, where damaged mitochondria contribute to further ROS production [197,198]. Mitochondrial dysfunction leads to reduced oxidative phosphorylation and decreased ATP production, which also promotes the overproduction of pro-inflammatory and pro-oxidative signals, deranging mitochondrial biogenesis and mitophagy [199]. In senescent cells, mitophagy is downregulated, leading to decreased PTEN-induced putative kinase 1 [200]. The mitochondrial DNA damage is repaired by the silent mating type information regulation 1 homolog-3 (SIRT3) gene; the damage of mitochondrial DNA and increase in ROS is observed when SIRT3 is depleted, where mtDNA released into the cytoplasm from mitochondria is sensed by cyclic GMP-AMP synthase (cGAS), which activates STING protein and promotes the senescence of lung fibrosis [201,202].

4.2. DNA Damage and Telomere Attrition

Among all macromolecules, DNA is the only one that is repaired rather than degraded upon damage, which implies a vital role in cell function. Exposure to environmental triggers and reactive oxygen species within the body cause DNA double-strand breaks and telomere attrition, which activate DNA-damage signaling pathways and alter transcriptional programs [203]. In the presence of catastrophic genotoxic stress, the cell undergoes apoptosis; however, initially, the DNA damage is sensed and triggers cell cycle arrest until it is repaired [204]. In COPD, chronic exposure to cigarette smoke contributes to oxidative stress, leading to telomere DNA damage, cellular senescence, and inflammation in the lung [205,206]. Damage to the sheltering complex TPP1 results in persistent telomeric DNA damage, leading to COPD and emphysema, which can be due to persistent exposure to oxidative stress-causing agents, such as cigarette smoke, that damage epithelial barriers [207]. The double-strand breaks (DSBs) of DNA caused by exposure to cigarette smoke contribute to COPD pathogenesis through activation of pro-inflammatory responses and apoptosis [194]. The sensor of double-stranded DNA breaks is phosphorylated histone H2AX, which recruits phosphorylated ATM/ATR and 53BP1 to the damage site, a process mainly observed in alveolar epithelial type 2 cells in COPD patients [194,208,209]. In a rodent model of bleomycin-induced pulmonary fibrosis, lung fibrosis is established through single- or double-stranded DNA breaks induced by bleomycin, leading to lung inflammation, where DNA damage induces senescence [210]. Telomeres are essential for cell fate and aging and are the short DNA repeats at the ends of chromosomes. These telomeres maintain the integrity of the chromosome; these telomeres are shortened with repetitive cell cycles, which further activates the DNA damage response [211]. Telomere shortening is counteracted by the enzyme telomerase, which has been found mutated in cases of pulmonary fibrosis. The absence of the telomerase reverse transcriptase gene increases the likelihood of cellular senescence and susceptibility to bleomycin-induced epithelial injury.

4.3. Chronic Inflammation and Inflammaging

The activity of Nuclear factor-kB (NF-kB) increases with age and age-related chronic diseases, and it is triggered by cell injury and inflammation [212,213]. In mammals, the NF-kB family consists of five related members, including NF-kB1 (p50), NF-kB2 (p52), RelA(p65), RelB, and RelC [214]. In senescence, SASP is regulated by the NF-kB signaling pathway [103]. The high activation of NF-kB signaling in COPD is related to epithelial cell senescence, which activates continuous production of cytokines including IL-6, IL-8, IL-1α / β, TNF-α, and CXCL [215,216]. Cell senescence is delayed when NF-kB signaling components, such as p65, are inhibited, leading to reduced SASP and reduced levels of pro-inflammatory factors IL-6 and IL-8 [217]. However, the increase in alveolar epithelial cell senescence is observed when NF-kB signaling is activated by CCAAT/ enhancer protein [218]. This clearly depicts the relation between NF-kB signaling and inflammaging. In lung samples from both bleomycin-induced pulmonary fibrosis in mouse models and idiopathic pulmonary fibrosis, elevated phosphorylated NF-kB was observed, linking the pathogenesis of IPF to the NF-kB signaling pathway [219,220]. On the other hand, inflammasomes are multimeric oligomers made of nucleotide-binding oligomerization domain receptors (NLR), apoptosis-associated speck-like protein containing CARD (ASC), and an enzymatic effector-caspase, which mainly work by generating biologically active cytokines, which are generated by the cleavage of precursor forms of IL-1β and IL-18 [221,222]. The NLRP3 is activated by various pathogens, lipopolysaccharides, lipo-oligosaccharides, and various exogenous toxins, which further activate caspase-1 and trigger production of IL-1β and IL-18, causing inflammation in the airways [223,224,225]. Leukocytosis and the release of neutrophils from the bone marrow are induced by IL-1β and are considered key drivers of airway inflammation in COPD and asthma [226]. During the senescence process, a large variety of proteins are released, contributing to the SASP, which includes cytokines, chemokines, and metalloproteinases. A pro-inflammatory cytokine such as IL-6 reinforces SASP release, thereby inducing senescence by binding to the CXCR2 receptor [88,100,112,227]. Kojima H et al. have demonstrated the newer model of IL-6/STAT3-induced senescence in TIG3 fibroblasts, where IGFBP5 (insulin-like growth factor-binding protein 5) is produced after activation of STAT-3 (signal transducer and activator of transcription 3) by IL-6, leading to the formation of reactive oxygen species, DNA Damage response (DDR), and SASP. Persistent activation of the IL-6/STAT3/IGFBP5 axis leads to constant production of ROS, SASP, and DDR, resulting in p53-dependent premature senescence [228].

4.4. Metabolic and Lipid Dysregulation

The senescent cells are metabolically active despite their inability to proliferate, with reduced ATP production due to decreased oxidative phosphorylation (OXPHOS), yet mitochondrial mass is high [229,230]. Inhibition or dysregulation of mitochondrial function, with inhibitors or by removal of mitochondrial chaperone heat shock protein A9 or sirtuin family members, induces senescence [231]. The physiological lipid metabolism is composed of three main pathways: fatty acid B-oxidation (FAO), fatty acid synthesis, and cholesterol synthesis. Fatty acid β-oxidation is increased in senescent bronchial epithelial cells in pulmonary diseases such as asthma; moreover, in COPD, the regulation of OXPHOS and prostaglandin E2 (PGE2), a lipid mediator, is reduced in pulmonary fibroblasts [232]. Moreover, macrophages have a dual role in regulating inflammation and metabolism, in which the FAO within macrophages determines their phenotype. The M2 macrophage has increased FAO and secretion of anti-inflammatory cytokines, supported by OXPHOS. The M1 macrophage is dominant in inflammatory lung diseases such as COPD, with low FAO leading to reduced fatty acid breakdown and increased fatty acid accumulation, resulting in lipid-laden macrophages that contribute not only to phagocytic function but also to inflammation and airway remodeling [233,234].

4.5. Extracellular Vesicles and Paracrine Senescence

Aging is a major risk factor for the development of chronic lung diseases, in which senescent cells within the lung produce various components, including microRNA, messenger RNA, DNA, and proteins, packaged into extracellular vesicles (EVs) [235]. These senescence-associated EVs mediate communication between various cells, leading to the induction of cellular senescence, inflammation, and cancer progression in both autocrine and paracrine manners to maintain homeostasis [236,237]. The senescence-associated EVs produced from bronchial epithelial cells on exposure to cigarette smoke extract (CSE) induced fibrosis via EV miR-210 in COPD [238]. In idiopathic pulmonary fibrosis (IPF), EVs composed of miR-21, which contribute to low-grade chronic inflammation, are more prevalent in patients with IPF than in controls [239,240]. Justine et al. reported that EVs derived from small airway epithelial cells from patients with COPD, enriched with miR-34a, induce senescence in healthy cells, as evidenced by elevated expression of senescence markers such as p21 and β-galactosidase activity, and reduced expression of SIRT1 [241]. Furthermore, Tsukasa et al. demonstrated the role of EVs in inducing senescence via a paracrine effect, showing that EVs derived from lung fibroblasts in idiopathic pulmonary fibrosis induced senescence in lung epithelial cells through activation of the DNA damage response. These EVs were enriched in miR-23b-3p and miR-494-3p, which reduced SIRT3 expression and promoted cellular senescence [242]. Collectively, these findings suggest that EVs are components of the SASP and possess pro-senescent properties, and may contribute to the pathophysiology and progression of age-related diseases.

5. Consequences of Cellular Senescence in Chronic Lung Disease

5.1. Persistent Inflammation and SASP Amplification

With age, senescent cells accumulate, where not only is the removal of senescent cells by immune cells impaired, but there is also increased production of pro-inflammatory cytokines, leading to persistent chronic inflammation [243]. Senescent cells influence surrounding and distant tissues via SASP, inducing inflammation and senescence. The SASP released by senescent cells comprises chemokines, cytokines, and proteases that attract and activate immune cells and induce their migration [116]. Chronic exposure to SASP can lead to chronic tissue inflammation and damage, in which even immune cells, such as eosinophils, neutrophils, macrophages, T cells, and B cells, undergo senescence and further contribute to chronic inflammation [244]. In COPD patients, the cytokines IL-6 and IL-8 are elevated and are likely considered SASP factors, contributing to the chronic inflammatory state in COPD [245,246,247]. In idiopathic pulmonary fibrosis, the SASP produced contains a variety of cytokines (IL-6, IL-1α, IL-1β), chemokines (CCL2, CXCL8), growth factors (TGF-β, WNT), and metalloproteases (MMP2, MMP12), which facilitate immune cell recruitment, inflammation, and ECM deposition [138,248]. Moreover, macrophages are functionally divided into two groups, namely M1 (pro-inflammatory) and M2 (immunosuppressive) macrophage polarization, which is influenced by SASP [249]. In bleomycin-induced lung fibrosis, the M2 macrophages and fibroblasts are activated, and the macrophage inhibitor cytokine-1 (MIC-1) is secreted by alveolar epithelial cells [250].

5.2. Impaired Tissue Repair and Regeneration

The lung is highly capable of repairing itself with a vast diversity of progenitor and stem cells in response to injury, and its failure to regenerate and stem cell exhaustion are consequences of cellular senescence. The quantity of alveolar type 2 epithelial cells (AEC2s) is maintained, unlike basal and club cells, which decrease with age, yet it exhibits defects in self-renewal and differentiation capacity [251]. However, the maintenance of AEC2 stem cells depends on mesenchymal cells, which indicates the importance of epithelial stem/progenitor cells. Moreover, bone marrow-derived mesenchymal stem cells contribute to tissue repair following lung injury, suggesting that mesenchymal stem cell senescence with age reduces their capacity to prevent fibrogenesis [252].

5.3. Structural Remodeling and Functional Decline

One of the most serious consequences of cellular senescence in chronic lung disease is alterations in the extracellular matrix (ECM), leading to a decline in lung function through structural remodeling of airways and lung parenchyma [253]. Airway remodeling and obstruction result from persistent low-grade chronic inflammation and increased growth factor secretion. Elevated smooth muscle mass, fibrosis, and angiogenesis can lead to loss of lung function, which is observed when senescent cells secrete extracellular matrix proteins and other factors [254]. The cells involved in airway remodeling in asthma are airway smooth muscle cells, epithelial cells, and fibroblasts, and the factors that enhance airway hyperresponsiveness and remodeling, such as TGF-β, EGF, and matrix metalloproteinases, are secreted by senescent epithelial cells [255]. In idiopathic pulmonary fibrosis, epithelial cells induce fibroblast proliferation, ECM production, basement membrane disruption, and airway remodeling via a large number of mediators, leading to ECM stiffness, loss of lung function, and worsened hypoxemia [96,256,257].

5.4. Susceptibility to Infection and Exacerbation

The low-grade chronic inflammation that occurs with aging is known as inflammaging, driven by declines in both innate and adaptive immunity. A dysregulated immune system, particularly when the macrophage population shifts predominantly towards M1-like (inflammatory) rather than M2 (phagocytic), is associated with an increased risk of infection due to impaired immune function and phagocytosis [258]. Moreover, with age, the production of antimicrobial peptides such as cathelicidin and B-defensin-2 is diminished with age [259]. On the other hand, ciliary movement is reduced with age, leading to decreased clearance of pathogens from the airway [260].

6. Senescence Across Chronic Lung Disease

6.1. Chronic Obstructive Pulmonary Disease (COPD)

The pathogenesis of COPD is mainly due to the persistent chronic low-grade inflammation leading to a reduction in lung function caused by obstruction of the airway [261,262]. Cigarette smoking is one of the major risk factors for the development of COPD, which induces oxidative stress on airway epithelial cells, which is a major contributor to cellular senescence [263]. Persistent oxidative stress due to cigarette smoke exposure leads to increased oxidative DNA damage in the lungs, which is characterized by elevated 8-hydroxy-2′-deoxyguanosine and γH2AX levels, and also activates p53/p21 and p16/pRB pathway [264,265]. A study by Yin Zhu et al. observed increased expression of cytochrome P450 family 1, subfamily B, member 1 (CYP1B1) following exposure to cigarette smoke, which contributes to CS-induced lipid accumulation in alveolar type II epithelial (AT2) cells in COPD [266]. Moreover, macrophages are observed in abundance in COPD lungs, where exposure to cigarette smoke induces the release of various molecules such as cytokines, chemokines, and proteases, which are involved in inflammatory processes and destruction of the extracellular matrix, leading to tissue destruction and progression of COPD [267,268]. The SASP profiles in COPD were observed with elevated levels of cytokines (IL-1 β, IL-5, TNF-α), chemokines (CCL2, CXCL1, CXCL8), and proteases (MMP-2, MMP-9) and growth factors like TGF-β, which implies that the senescence contributes to the development of COPD due to chronic inflammation accompanied by thickening of airway walls and luminal stenosis [269,270].

6.2. Idiopathic Pulmonary Fibrosis (IPF)

Idiopathic pulmonary fibrosis is a fatal disease characterized by irreversible damage to the alveolar structure and dysregulated extracellular matrix deposition, which may result from the senescence of various pulmonary cells, such as Type II alveolar epithelial cells and fibroblasts/myofibroblasts [126]. Physiologically, the body balances between wound healing and fibrosis resolution, which involves inflammation, fibroblast migration, proliferation, epithelial injury, matrix deposition, and remodeling; however, in the geriatric population, this imbalance leads to the development of fibrosis and organ damage. The mesenchymal cells are activated and recruited due to inadequate restoration of epithelial cells, ultimately leading to fibrogenesis by fibroblasts and myofibroblasts, where α-smooth muscle actin is newly produced [271,272]. The senescent alveolar type II epithelial cells and lung fibroblasts adopt a profibrotic SASP, respectively, leading to ECM deposition by TGF-β, IL-6, MMP12, IL-8, and TNF-α [138,273,274], which exhibits a strong association between cellular senescence and the fibrotic process.

6.3. Pulmonary Hypertension and Vascular Disease

Pulmonary arterial hypertension is a rare vascular disease that affects the pulmonary vasculature, characterized by dysfunction of pulmonary artery endothelial cells and smooth muscle cells, and chronic inflammation of blood vessels, leading to elevated pulmonary arterial blood pressure, with senescence of these cells observed [275,276]. The activation of DNA Damage response (DDR), p53, p21, p16 signaling, mitochondrial dysfunction, telomere erosion and pro-inflammatory SASP such as IL-6, IL-1 β, TGF-β is observed due to increased vascular resistance which is triggered from the senescent pulmonary artery endothelial cells [277,278,279], whereas, pulmonary artery smooth muscle cells become senescent due to mitochondrial injury-induced increases in ROS which further leads to vascular remodeling contributing to the development and progression of PAH [280].

6.4. Other Chronic Lung Conditions

Bronchial asthma is characterized by cough associated with sputum, exacerbations, and irreversible dilatation of bronchi. The chronic inflammatory state might be due to senescent airway epithelial cells with elevated γH2AX, p21 signaling, and telomerase-associated DNA damage, suggesting that it is a feature of bronchiectasis [281]. Another common chronic lung disease is asthma, which is expected to affect anyone, irrespective of age group. Asthma in elderly patients is more severe. Usually, immune cells are associated with the pathogenesis of asthma; however, elevated oxidative stress in neutrophilic asthma also drives senescence in airway epithelial cells [282]. IL-6 and CXCL-8 are considered mediators of neutrophilic asthma, which also can induce senescence and are a part of SASP signals [283]. T2 immunity is driven by thymic stromal lymphopoietin, which is produced by epithelial cells in asthmatic patients. It also induces senescence with elevated expression of p21, p16, and SA-β-gal staining. p53, p21, and SASP expression is elevated in airway smooth muscle cells [254,284].

7. Therapeutic Opportunities Targeting Senescence

Cellular senescence can be targeted by various therapeutic strategies, including senolytic drugs, senomorphic approaches, immune-mediated clearance of senescent cells, and emerging treatment strategies.

7.1. Senolytic Therapy

Senolytic drugs eliminate senescent cells, which have activated pathways such as p21, p53, and Bcl2 family members without affecting the normal cells.

• BCL-2 family inhibitors:

BCL-2 is a family of antiapoptotic proteins, including BCL-2, BCL-XL, and BCL-W, expressed in senescent cells to protect the cells from apoptosis and facilitate the survival of senescent cells. ABT-263 (Navitoclax) is a senolytic agent that eliminates senescent pulmonary fibroblasts and senescent pulmonary vascular endothelial cells via apoptosis and thereby decreases the progression of fibrosis and reverses pulmonary arterial hypertension by reversing pulmonary artery remodeling, respectively, in mice [285,286,287]. However, thrombocytopenia and neutropenia are the major side effects. The conjugation of Navitoclax with galactose or proteolysis-targeting-chimeras will reduce the risk of side effects by selectively targeting senescent cells with SA-β-Gal and destroying BCL-2 family protein-containing senescent cells via ubiquitination, respectively, thereby reducing hematological side effects [288,289,290]. ABT-737 has significantly reduced markers of senescence in lungs induced by ionizing radiation and airway inflammation, suggesting it could be a promising therapy for asthma [291,292].

• Dasatinib-Quercetin:

Dasatinib is a tyrosine kinase inhibitor, and quercetin is a dietary flavonoid that inhibits BCL-2 and PI3K signaling in senescent cells, thereby eliminating senescent cells, reducing the expression of p16 levels and SASP response [243]. Dasatinib plus quercetin (D + Q), when administered orally thrice a week for 3 weeks to 12 IPF patients in a randomized controlled trial, no improvement in lung function was observed, but there was an increase in adverse effects; however, it decreased fibrosis and limited the lung impairment in fibrotic mouse models [138,293].

Despite substantial preclinical evidence supporting the notion that senescent-cell elimination is not totally beneficial and appears to depend strongly on cell type, disease stage, and the temporal context of senescence. This discrepancy is already apparent in IPF. Although a study showed that D + Q reported improvements in physical performance measures, pulmonary function remained unchanged [294]. Another randomized pilot study indicates that D + Q did not show meaningful differences in pulmonary function, physical function, or frailty compared with placebo [293]. Similarly, successful pharmacological depletion of senescent-like myeloid cells with D + Q in experimental autoimmune encephalomyelitis failed to improve clinical disease, demyelination, or axonal degeneration, and neither increased D + Q exposure nor genetic clearance of p16-expressing cells improved these outcomes [295]. These observations indicate that reducing senescent-cell abundance alone may be insufficient when senescence is not the dominant driver of disease or when irreversible structural damage has already occurred. On the other hand, senescence may also exert beneficial effects during acute tissue injury. p16-expressing fibroblasts served as an inflammation-responsive reparative niche and promoted lung epithelial regeneration [296]. Similarly, activation of the p53/p21 pathway during acid aspiration and mechanical ventilation induced early senescence-associated features while limiting apoptosis and acute lung damage; conversely, Cdkn1a deficiency reduced the senescence response but exacerbated lung injury [297]. Collectively, these findings support that the senescence response can facilitate damage containment and tissue regeneration, whereas persistent senescence and prolonged SASP activity favor chronic inflammation, fibrosis, and regenerative failure.

7.2. Senomorphic Drugs

These drugs act by modulating SASP released from senescent cells, without eliminating senescent cells through apoptosis [298].

• SASP suppression:

a. mTOR pathway inhibitors:

Rapamycin, also known as sirolimus, inhibits the mTOR signaling pathway, which typically regulates cellular senescence, metabolism, and autophagy. It attenuates senescence-associated diseases and extends the lifespan of mice, as evidenced by reduced SA-β-gal staining and reduced levels of SASP mediators with a low dose of rapamycin [299].

b. Nuclear factor-kB inhibitors:

NF-kB activates senescence, DNA damage, and regulates SASP, which are halted in the presence of an inhibitor or genetic knockdown. SR12343, an NF-kB inhibitor, inhibits or reduces cellular senescence in human cells and animal models [212,300,301].

c. JAK/STAT inhibitors:

The JAK1/2 inhibitor ruxolitinib inhibits the activated JAK/STAT pathway in senescent cells, leading to reduced cellular senescence via SASP suppression, as observed in human senescent fibroblasts and mouse models [302]. Bleomycin-induced lung inflammation and fibrosis are reduced in the presence of JAK2 inhibitor fedratinib [303].

• Metabolic and mitochondrial targeting:

mTORC1 is a regulator of senescence, and AMP Kinase (AMPK) is an endogenous, naturally produced mTORC1 inhibitor that reduces cellular senescence [304]. Moreover, metformin, an AMPK activator, attenuates elastase-induced emphysema in mice by reducing the SASP response in airway epithelial cells [305]. Other agents, such as sirtuins, which are considered NAD+ dependent protein deacetylases, regulate cell metabolism, inflammation, oxidative stress, and senescence [306]. SIRT1 and SIRT6 are involved in DNA repair and antioxidant expression. However, SIRT1 is primarily involved in maintaining mitochondrial function, inhibiting p53-induced senescence, and activating NF-kB, whereas SIRT6 regulates metabolic homeostasis and telomere length, ultimately regulating senescence [307,308].

7.3. Immune-Mediated Clearance of Senescent Cells

Senescent cells have their own senescent cell antiapoptotic pathways to protect themselves from the pro-apoptotic SASP, which are later removed by immune cells, a process known as immune-mediated clearance. However, with age, the burden of senescent cells is increased due to their accumulation, which requires treatment to enhance immune-mediated clearance [309]. The accumulation of senescent cells can result from either impaired clearance by immune cells or exhausted immune cells that fail to recognize senescent cells.

• Natural Killer cell-based mechanism

These natural killer (NK) cells clear senescent cells by either damaging cell membranes with enzymes or by detecting membrane receptors [310]. The hepatic stellate cells (HSCs) in the hepatic fibrosis mouse model have increased expression of p21, p16, and SA-β-gal accumulation, which are detected by NKG2DA and DNAM1 receptors of NK cells, leading to the clearance of senescent-like HSCs and thus fibrosis via perforin exocytosis [311,312].

• Macrophage-based mechanism

Circulating monocytes or tissue-resident macrophages are attracted by SASP factors, such as MCP-1, MIP-1α, and GM-CSF, thereby enhancing phagocytosis. The cells are attacked by monocytes, which produce cytotoxic molecules such as ROS, TNF-α, and nitric oxide, and lack CD47 (CD47 provides a “don’t eat me” signal) via TLR signaling [302,313].

• Enhancing senescent cell recognition

The clearance of senescent cells has benefited age-related diseases, yet has affected healthy cells, which can be overcome with therapies that target specific senescent cells via anti-aging immunity vaccines. The senescent cell-specific molecules or overexpressed molecules are known as “seno-antigens” and can be recognized by the immune system and clear senescent specific cells [314]. The ability to recognize senescent cells and promote their clearance by immune cells declines with age due to immunosenescence, which can be enhanced using vaccination against seno-antigens. Cao et al. developed an anti-aging vaccine that delivers a combination of a cationic protein (CP) and seno-antigen peptides (Gpnmb), further activates CD8+T cells via a dendritic cell vaccine, and promotes clearance of senescent cells with an enhanced immune response [315].

7.4. Emerging and Experimental Therapies

The EVs carry a variety of cellular information and components, which can be used for the diagnosis and treatment of lung diseases. EVs have a special therapeutic potential against COPD, pulmonary fibrosis, interstitial pneumonia, and asthma [316,317]. The EVs derived from human bronchoepithelial cells have therapeutic potential for pulmonary fibrosis and lung epithelial cell senescence by inhibiting TGF-β-WNT signaling [318]. The EVs derived from mesenchymal stem cells promote macrophage polarization and wound angiogenesis and inhibit scar formation by inhibiting extracellular matrix accumulation [319].

8. Challenges, Knowledge Gaps, and Future Directions

A wide variety of cells are involved in lung senescence, contributing to chronic lung diseases such as pulmonary fibroblasts, alveolar epithelial cells, pulmonary smooth muscle cells, and pulmonary vascular endothelial cells, each contributing to the onset and progression of IPF, COPD, asthma, and PAH, respectively. Hence, the SASP produced is largely dependent on the heterogeneous nature of senotypes, which vary by cell type and might depend on the stage of disease progression. Hence, cell- and disease-stage-specific interventions are urgently needed to either cure or slow disease progression. Moreover, although there is increased research on the treatment of chronic lung diseases via senescence-targeted therapy, there is an absence of biomarkers for detecting lung senescence, which appears to be a translational barrier. Lung-specific senescence biomarkers are much needed to specifically identify pathologic senescent cells from physiologic senescent cells, providing a roadmap for the development of targeted therapies [320]. Lung-specific biomarkers can be identified through multi-omics profiling and single-cell technologies, which provide comprehensive insights into the characteristics of lung-resident senescent cells. The heterogeneity of senescent cells across different tissues and disease states can be elucidated using advanced approaches such as transcriptomics, epigenomics, proteomics, and metabolomics. These technologies facilitate the development of comprehensive senescent cell atlases, enabling the discovery of disease-specific biomarkers. Furthermore, integrating multi-omics datasets with machine learning and artificial intelligence approaches may reveal senescence-associated pathways and predictive biomarkers, thereby enhancing our understanding of disease mechanisms and supporting the development of precision diagnostic and therapeutic strategies.

Despite the route of administration of senotherapeutics in current clinical trials being systemic, pulmonary administration of senotherapeutics seems to be a safer choice, as systemic administration targeting lung diseases results in lower concentration in the lungs, where inhalational delivery of senotherapeutics is expected to have lower systemic exposure, fewer off-target side effects, and higher concentration of drugs in the target tissue. However, altered lung physiology in chronic respiratory diseases, including mucus hypersecretion, airflow obstruction, and dysregulated alveolar macrophage function, may significantly influence the pharmacokinetics and therapeutic efficacy of inhaled drugs. Furthermore, the pharmacokinetic and pharmacodynamic (PK/PD) profiles of inhaled senotherapeutics remain poorly characterized. Another major challenge is optimizing dosing strategies. For senolytic therapies, an intermittent “hit-and-run” dosing approach is currently considered advantageous because it reduces the burden of senescent cells while potentially minimizing adverse effects. However, the optimal dose and dosing frequency are likely to vary across different diseases and stages of disease progression and have not yet been fully established. Finally, although many preclinical and early clinical studies have demonstrated the beneficial effects of senotherapeutics, most have involved relatively short follow-up periods. Because chronic lung diseases often require prolonged treatment, long-term studies with extended follow-up are essential to evaluate sustained efficacy, establish long-term safety, and define the overall risk-benefit profile of these therapies.

9. Conclusions

Cellular senescence is considered a physiological protective mechanism in response to acute triggers. Persistent accumulation of senescent cells leads to chronic low-grade inflammation, tissue remodeling, and impaired regeneration, which ultimately contribute to the onset or progression of chronic lung diseases. A variety of treatments against senescence are largely non-specific; rational drug design guided by molecular profiling may enable the development of targeted, cell- and stage-specific therapies. Treatment approaches tailored to the cell type involved and the etiology of senescence, including lineage-specific approaches, can be expected to target the specific senescent cell subtype.