Cell Senescence and Fibrotic Lung Diseases

Abstract

Idiopathic pulmonary fibrosis (IPF) is a progressive fatal lung disorder with an unknown etiology and very limited therapeutic options. The incidence and severity of IPF increase with advanced age, suggesting that aging is a major risk factor for IPF. The mechanism underlying the aging-related susceptibility to IPF, however, remains unclear. Cellular senescence, a permanent arrest of cell growth, has been increasingly recognized as an important contributor to aging and aging-related diseases, including IPF. Senescent cells have been identified in IPF lungs and in experimental lung fibrosis models. Removal of senescent cells pharmacologically or genetically improves lung function and reverses pulmonary fibrosis induced by different stimuli in experimental fibrosis models. Treatment with senolytic drugs also improves clinical symptoms in IPF patients. These intriguing findings suggest that cellular senescence contributes importantly to the pathogenesis of fibrotic lung diseases and targeting senescent cells may represent a novel approach for the treatment of fibrotic lung disorders. In this mini review, we summarize the recent advance in the field regarding the role of cellular senescence in fibrotic lung diseases, with a focus on IPF.

Cellular Senescence

Cellular senescence, including replicative senescence (RS) and stress-induced premature senescence (SIPS), is a permanent arrest of cell growth. Cellular senescence, although linked to aging, can occur at any life stage from embryo to adulthood^{1, 2}. Cellular senescence is a highly regulated process and has both beneficial (fetal development and tumor suppression) and detrimental (premature aging-related diseases) effects, when this process is perturbed². Senescent cells are characterized by macromolecule damage, irreversible cell cycle arrest, altered metabolism and cell shape, and secretion of various pro-inflammatory cytokines, chemokines, growth factors, a feature best known as senescence associated secretory phenotype (SASP)^{1, 3, 4}. It has been well documented that the number of senescent cells increases with age and in aging-related diseases. The mechanisms responsible for cellular senescence under either physiological or pathological conditions remain poorly understood, although various hypotheses, including telomere shortening, oncogene activation, oxidative stress, DNA damage, and mitochondrial dysfunction have been put forward (Fig 1).

Fig 1. Schematic flow chart of cause and consequence of cell senescence.

SASP, senescence associated secretory phenotype.

It has been long recognized that cellular senescence contributes importantly to the aging-related decline in tissue regeneration capacity and the pathogenesis of aging-related diseases, including IPF^3–8. The mechanism whereby senescent cells contribute to aging and aging-related diseases, however, remains unclear. Progenitor cells or stem cells are particularly sensitive to senescence stresses. Therefore, one of the hypotheses is that cellular senescence leads to exhaustion of progenitor cells or stem cells, which, in turn, causes a decline in tissue regenerative capacity during aging or upon injury. Another potential mechanism whereby senescent cells contribute to aging and aging-related diseases is through secretion of pro-inflammatory cytokines/chemokines/growth factors (SASP). These biologically active molecules can exert profound effects on themselves (autocrine function) or on neighboring cells (paracrine functions). It has been reported that the SASP can spread senescence signals to surrounding cells, contribute to persistent inflammation, and induce profibrotic phenotypic changes of fibroblasts or macrophages^9–12. The third potential mechanism whereby cell senescence contribute to the development of lung fibrosis is protecting myofibroblasts, the major producer of extracellular matrix proteins, from apoptosis (Fig 1). Despite an incomplete understanding of the mechanism by which senescent cells contribute to aging and aging-related diseases, therapeutic approaches targeting senescent cells for the improvement of healthy aging and for the treatment of aging-related diseases have been growing rapidly^13–18. Further elucidation of the mechanisms underlying cell senescence and the mechanisms whereby senescent cells contribute to the pathogenesis of the diseases may lead to the discovery of novel therapeutics for the treatment of these aging-related diseases.

Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis is a progressive fatal lung disorder with unknown etiology. The disease is characterized by an excessive deposition of extracellular matrix proteins in the lung interstitium, leading to the destruction of lung structure and function. The median survival of IPF is 2–3 years after diagnosis and the mortality rate of IPF surpasses many types of cancers. There is a very limited therapeutic option for this devastating disease due to an incomplete understanding of its etiology and pathogenesis. Several risk factors have been postulated, including gene mutations, cigarette smoking, occupational exposure, and infectious agents; but the strongest link with IPF clinical disease expression/progression is aging^19–23. Therefore, IPF is considered a disease of aging^{6, 21, 22, 24}. Despite intensive studies, the mechanism underlying the aging-related susceptibility to IPF remains unclear. A current disease paradigm is that lung fibrosis develops as a result of unremitting insults plus genetic and aging-related risk factors, leading to alveolar epithelial cell injuries. Injured epithelial cells then secrete an array of growth factors, chemokines/cytokines, and proteases, which promote the activation of myofibroblasts and replacement of injured alveolar epithelium with fibrotic tissue, due to a decreased reparative capacity of the alveolar epithelium (senescence). Therefore, elucidation of the mechanism underlying cell senescence in lung cells, especially epithelial cells, may be a key for our understanding of the disease pathogenesis and for the development of effective treatments for this devastating disease.

Cell Senescence in IPF

Emerging evidence suggests that cellular senescence contributes importantly to the pathophysiology of IPF. Various types of cells, including alveolar epithelial type II cells, fibroblasts, and endothelial cells, have been shown to undergo senescence in IPF lungs^{14, 25–35}. Senescent cells have also been found in experimental lung fibrosis models induced by different stimuli^{12, 35–41}. Most importantly, studies have shown that elimination of senescent cells pharmacologically or genetically attenuates lung fibrosis and restores the lung function in mice with experimental lung fibrosis^{13, 14, 30}. A pilot clinical study further showed that senotherapy significantly and clinically-meaningfully improved the physical function evaluated as 6-min walk distance, 4-m gait speed, and chair-stands time in IPF patients¹⁸. Together, the data suggest that cellular senescence may underlie the pathophysiology of IPF and targeting senescent cells may be effective for the treatment of IPF.

Clinical findings:

Different types of cells, including alveolar epithelial type II cells, fibroblasts, and endothelial cells, have been shown to undergo senescence in IPF lung^{14, 25–35}. ATII cells can self-renew and can also differentiate into type I alveolar epithelial cells and therefore are considered as alveolar progenitor cells^{42, 43}. There is overwhelming evidence showing that alveolar epithelial cells in IPF lung undergo senescence 5, 14, 25, 28, 30, 35. Using immunostaining as well as x-gal staining techniques, several investigators have shown that epithelial cells in IPF lung express higher levels of cell cycle repressors p21, p16, and/or p53 and have higher activity of senescence associated beta-galactosidase (SA-β-gal), as compared to ATII cells in control lungs^{14, 25, 28, 30, 35}. Using single cell RNA sequence techniques, Reyfman et al. identified rare cell populations, including airway stem cells and senescent epithelial cells, in IPF lungs³¹. These senescent epithelial cells secrete various cytokines and chemokines related to SASP³³. Importantly, many alveolar epithelial cells in IPF lungs are also positive for TUNEL staining, a classical method to detect apoptotic cells, indicating that senescent epithelial cells may be prone to apoptosis^{25, 44}.

Fibroblasts are the primary producers of extracellular matrix proteins and play a central role in the development of lung fibrosis. Several studies have shown that fibroblasts in IPF lungs exhibit senescent phenotype, having an increased level of DNA damage, expressing higher levels of cell cycle repressors p21, p16, and/or p53, demonstrating positive SA-β-gal staining, and secreting various SASP cytokines^{26, 27, 29, 45, 46}. These senescent fibroblasts also express higher levels of myofibroblast marker alpha smooth muscle actin (α-SMA) and produce more extracellular matrix proteins^{26, 27, 29}. Interestingly, Yanai et al. reported that fibroblasts from IPF lungs exhibit accelerated replicative cellular senescence phenotype but are resistant to oxidative stress-induced cellular senescence²⁷. In contrast to senescent epithelial cells, senescent fibroblasts from IPF lungs are highly resistant to oxidative stress-induced cytotoxicity or apoptosis^{26, 27, 29}.

Besides ATII cells and fibroblasts, other types of cells in IPF lung also show a senescent phenotype. Increased expression of p21 and activity of SA-β-gal are detected in bronchial epithelial cells in IPF lungs 5. It has also been reported that bone-marrow-derived mesenchymal stem cells (B-MSCs) from IPF lungs have an increased expression of p21, enhanced activity of SA-β-gal, and a higher level of DNA damage as well as a decreased differentiation capacity, compared to fibroblasts from age-matched healthy controls¹¹. Incubation of normal-aged fibroblasts with the conditioned media from senescent IPF B-MSCs stimulates the expression of p21 and p53 as well as collagen genes in these normal fibroblasts, suggesting that these senescent fibroblasts have a paracrine pro-senescence capacity¹¹.

Experimental lung fibrosis models:

Senescent cells have also been detected in the lung of experimental fibrosis models^{12, 35–41}. Bleomycin is an anti-cancer drug with a side effect of lung fibrosis. Therefore, bleomycin-induced lung fibrosis has been widely used to study the pathogenesis of lung fibrosis and the therapeutic potential of anti-fibrosis candidate drugs. It has been well-documented that intratracheal instillation of bleomycin induces senescence in alveolar epithelial cells and lung fibroblasts in mice, associated with lung fibrosis^{35, 37, 38, 41, 47, 48}. Radiation-induced lung fibrosis is another commonly used experimental lung fibrosis model. It has been reported that the number of ATII cells that express p21 and stained positive for SA-β-gal is significantly increased in mice exposed to thoracic radiation, compared to unexposed mice³⁹. Treatment with NADPH oxidase inhibitor diphenyleneiodonium (DPI) attenuates radiation-induced ATII cell senescence as well as lung fibrosis³⁹, suggesting that ATII cell senescence may contribute to radiation-induced lung fibrosis.

Although it has been well documented that ATII cells undergo senescence in IPF lung and in experimental fibrosis models, how senescent ATII cells contribute to the development of lung fibrosis remains poorly understood. Emerging evidence suggests that senescent cells may affect the functions of nearby cells through secretion of biological active molecules such as cytokines, chemokines, growth factors, and proteases. Consistent with this hypothesis, studies have shown that senescent alveolar epithelial cells secreted IL-6, TNF-α, MMP-2, and MMP-9^33,38. Treatment of ATII cells with bleomycin in vitro also induced cell senescence and promoted the SASP³⁵. Transforming growth factor beta 1 (TGF-β1) is one of the most ubiquitous and potent profibrogenic cytokines. Our recent studies showed that treatment with TGF-β1 under the conditions that did not cause apoptosis induced ATII cell senescence and stimulated secretion of pro-inflammatory and pro-fibrotic cytokine/growth factors in vitro¹². Incubation of primary alveolar macrophages with the conditioned media from the senescent ATII cells promoted a profibrotic phenotype of macrophages¹². Similarly, Tian et al. reported that culture of lung fibroblasts with the conditioned media from senescent ATII cells stimulated collagen deposition in fibroblasts³⁵. Together, these observations suggest that senescent ATII cells may promote lung fibrosis partially by secreting biologically active molecules that promote a profibrotic phenotype of adjacent fibroblasts and alveolar macrophages^{12, 35}.

Molecular Mechanisms Underlying Cellular Senescence in IPF Lungs

The mechanisms underlying cell senescence in either physiological or pathological conditions remain unclear. Although telomere shortening is believed to be responsible for replicative senescence in proliferating cells, the molecular mechanism linking telomere shortening to cell senescence is still not well defined. Furthermore, although it has been well documented that various stress conditions, including DNA damage and oxidative stress, induce premature cell senescence, the signaling pathways leading to cell senescence under these different stress conditions remain poorly understood. In this section, we summarize recent findings regarding the potential mechanisms underlying lung cell senescence in IPF (Fig 2).

Fig 2. Potential mechanisms underlying cell senescence in IPF lung.

TERT, telomerase reverse transcriptase; PARN, polyadenylation-specific ribonuclease deadenylation nuclease; RTEL1, telomere elongation helicase 1; ROS, reactive oxygen species; PAI-1, plasminogen activator inhibitor 1.

Telomere shortening and cell senescence in IPF lungs:

A telomere is a region of tandem repeats of short DNA sequences at the end of a chromosome, which protects the chromosome from erosion. During the cell cycle, an incomplete DNA replication leads to the loss of a part of the telomere and thereby instability of the chromosome. Telomerase, consisting of telomerase reverse transcriptase (TERT) and telomerase RNA (TERC), on the other hand, restores the telomere length. It has been well documented that telomere length is shortened and DNA damage is increased in IPF lungs, compared to controls^49–53. It has been reported that 15 % of familial IPF patients and 3% of sporadic IPF patients bear a loss of function mutation in TERT, whereas 30% of familial IPF and 25%−50% of sporadic IPF have telomere shortening^{49, 50, 54, 55}. These results suggest that TERT mutations only account partially for the telomere shortening in IPF. A recent study from sequencing exome of familial kindreds with pulmonary fibrosis showed that mutations in polyadenylation-specific ribonuclease deadenylation nuclease (PARN) and regulator of telomere elongation helicase 1 (RTEL1) are associated with telomere shortening and familial pulmonary fibrosis⁵². Cigarette smoking is one of the risk factors for IPF. Cigarette smoking has been shown to induce telomere shortening^56–59, although the findings are not always consistent⁵⁹. It has also been reported that TERT mutation carriers may be more sensitive to cigarette smoke induced pulmonary fibrosis⁶⁰. Individuals with short telomeres and/or known telomere-related mutations have higher levels of fibrotic tissues and show more rapid disease progression and shorter lung transplant–free survival^{53, 61, 62}, further suggesting a link between telomere shortening and IPF pathophysiology.

Experimentally, several studies have shown that deletion of telomerase or inducing severe telomere dysfunction leads to cell senescence and lung fibrosis in animal models^63–66. Chen et al. reported that knockout of mouse gene Terc or Tert caused pulmonary alveolar stem cell replicative senescence, epithelial impairment, lung inflammation, myofibroblast differentiation, and lung fibrosis⁶⁴. Naikawadi et al. showed that deletion of telomeric repeat-binding factor 1 (TERF1 or TRF1) specifically in ATII cells resulted in telomere shortening, epithelial cell senescence, and lung fibrosis⁶⁵. Deletion of TRF1 in collagen expressing cells, however, cause pulmonary edema, not fibrosis, suggesting that telomere shortening in ATII cells, but not in collagen-expressing cells, is associated with lung fibrosis⁶⁵. Liu et al. further showed that specific deletion of the Tert gene in ATII cells in mice did not lead to lung fibrosis; however, these mice were more susceptible to bleomycin-induced ATII cell senescence and lung fibrosis⁶⁶. As telomere shortening is closely related to cell senescence, whereas cell senescence is a hallmark of aging, it is hypothesized that one of the mechanisms underlying lung cell senescence and thus aging-related susceptibility to IPF is telomere shortening^{49, 65, 67–69}.

Epigenetic mechanisms of cell senescence in IPF:

Epigenetic modifications are heritable changes in the expression of genes without involving in DNA sequence [28]. The most important epigenetic mechanisms are DNA methylation, histone post-translational modifications, and non-coding RNA-mediated gene silencing⁷⁰. Accumulated evidence indicates that epigenetic modifications, including both DNA methylation and histone modifications, are involved in cellular senescence in fibrotic lung diseases^{5, 71, 72}. Sanders et al. reported that senescent fibroblasts had global increases in histone (H4K20) trimethylation and decreases in histone (H4K16) acetylation⁷². These fibroblasts also had marked enrichment of the Bcl-2 gene that was H4K16 acetylated and H4K20 demethylated. These data suggest that these epigenetic modifications led to active transcription of the Bcl-2 gene, which renders senescent fibroblasts an increased resistance to apoptosis⁷¹. Sanders et al. also reported that the NADPH oxidase 4 (Nox4) gene, which codes a reactive oxygen species generating enzyme involved in fibrogenesis, is enriched with the activation histone marker, H4K16Ac, but deprive of the repressive histone mark, H4K20Me3, suggesting that epigenetic mechanisms are involved in the regulation of this gene⁷².

PAI-1, p53, and cell senescence in IPF lungs:

p53, a tumor repressor, is a cell cycle repressor and a master controller of cell senescence and apoptosis. The p53 pathway is among the most dysregulated pathways identified in IPF lungs^{29, 41, 73}. Although it is well known that DNA damage leads to increased stability and activation of p53 protein through a variety of posttranslational modifications in cancer cells, how p53 is activated in lung cells in fibrotic diseases is unclear^{74, 75}. In a recent study, Zhang et al. reported that interleukin-18 (IL-18) promotes fibroblast senescence in fibrotic lung through down-regulation of Kloth expression, which is associated with increased expression of cell cycle repressor p53, suggesting that increased IL-18 may be one of initial signals inducing p53 in senescent fibroblasts in fibrotic lung disease⁷⁶.

Plasminogen activator inhibitor 1 (PAI-1) is a primary inhibitor of tissue type and urokinase type plasminogen activators (tPA and uPA), which convert plasminogen into plasmin, a serine proteinase that plays an essential role in fibrinolysis. In addition to blocking fibrinolysis through inhibition of tPA and uPA, PAI-1 is also involved in the regulation of many other biological processes, including degradation of extracellular matrix proteins, cell adhesion, migration, proliferation, and apoptosis⁷⁷. PAI-1 expression increases with age in the plasma of human beings, in klotho (kl/kl) mice, a murine model of aging, and in many aging-related pathological conditions including IPF^78–84. Studies, including ours, suggest that increased PAI-1 expression plays a key role in the development of lung fibrosis, although the underlying mechanism remains elusive^{41, 85–88}.

Notably, increased PAI-1 expression has long been used as a marker of replicative and stress- induced cellular senescence. Emerging evidence, including that from our studies, further suggests that PAI-1 is not merely a marker but also a key mediator of cell senescence both in vitro and in vivo^{12, 41, 89–93}. Chung et al. reported that intraperitoneal injection of a truncated PAI-1 protein significantly attenuated irradiation-induced pneumocyte senescence and lung fibrosis in mice⁹¹. In a recent study, we showed that bleomycin instillation increased PAI-1 expression and induced ATII cell senescence as well as lung fibrosis in mice⁴¹. Specific ablation of the PAI-1 gene in ATII cells in mice significantly attenuated bleomycin-induced p53 and p21 expression and stimulated phosphorylation of retinoblastoma protein (pRb) in these cells in mice⁴¹. This is associated with a reduction in lung fibrosis⁴¹. Silencing p53, on the other hand, eliminated PAI-1 protein-induced senescence in rat lung epithelial cells in vitro⁴¹. Together, our data suggest that increased PAI-1 mediates bleomycin-induced ATII cell senescence through activation of p53-p21-pRb cell cycle repression axis and that ATII cell senescence contributes importantly to bleomycin-induced lung fibrosis. In another study, we showed that TGF-β1 induced ATII cell senescence through induction of p16, not p53¹² and that senescent ATII cells secreted numerous pro-inflammatory and pro-fibrotic cytokines/chemokines, which promoted a pro-fibrotic phenotype in alveolar macrophages¹². As PAI-1 expression is increased with age and in IPF, it is suggested that increased PAI-1 expression may underlie ATII cell senescence in IPF lungs.

The mechanism underlying increased PAI-1 expression in senescent cells in fibrotic lung diseases and the mechanism whereby PAI-1 promotes p53 expression are not well understood. Previous studies from this lab and from others have shown that reactive oxygen species (ROS) or oxidative stress induces PAI-1 in cultured cells and in vivo. As oxidative stress is increased in fibrotic lung diseases and many profibrotic mediators stimulate ROS production, it is believed that one of the mechanisms underlying increased PAI-1 expression in fibrotic lung is increased oxidative stress. Our previous studies have also shown that PAI-1 mediates bleomycin-induced phosphorylation of p53 at serine 18 in mouse lung tissue (equals serine 20 in human p53)⁴¹. As increased phosphorylation of p53 at serine 18 in mouse or at serine 20 in human prevents the binding of p53 to murine double minute 2 (MDM2), a major E3 ubiquitin ligase involved in p53 degradation⁹⁴, it is hypothesized that PAI-1 increases p53 protein abundance in alveolar epithelial cells probably by stabilizing p53 protein. More studies are needed to prove this hypothesis and to reveal the molecular mechanism whereby PAI-1 modulates p53 phosphorylation.

microRNAs and cell senescence in IPF lung:

MicroRNAs (miRNAs) are small single strain noncoding RNAs with an average length of around 18–22 nucleotides. miRNAs regulate gene expression through inducing degradation of target mRNAs or inhibiting the translation of target mRNAs by binding to their 3’-untranslated regions. Emerging evidence indicates that miRNAs are involved in the regulation of cell senescence in various pathological conditions, including IPF^{28, 47, 95–97}. Disayabutr et al. reported that miR-34a, miR-34b, and miR-34c were significantly elevated in alveolar type II cells in IPF lungs, compared to patients with other interstitial lung diseases or normal controls²⁸. This was associated with increased expression/activity of senescence markers p16, p21, p53, and SA-β-gal, and decreased expression of miR-34 target genes, including E2F1, c-Myc, and cyclin E2²⁸. Overexpression of miR-34a, miR-34b, or miR-34c in lung epithelial cells also induced cell senescence²⁸. These results suggest that increased expression of miR-34a, miR-34b, and miR-34c may contribute to ATII cell senescence in IPF lungs.

Our studies further showed that, although miR-34a promoted senescence in both fibroblasts and alveolar epithelial cells (AECs) in bleomycin-induced lung fibrosis model, the role of miR-34a in lung fibrosis depended on the age of mice^{47, 98}. miR-34a expression was significantly increased, mainly in myofibroblasts, in the lung of bleomycin challenged young mice⁹⁸. Deletion of miR-34a augmented the sensitivity of young mice to bleomycin-induced lung fibrosis⁹⁸. This was associated with diminished senescence and apoptosis in lung fibroblasts. These results suggest that miR-34a protects young mice from bleomycin-induced lung fibrosis at least in part by inducing lung fibroblast senescence and apoptosis⁹⁸. In another study, we showed, however, that miR-34a was increased in AECs, but not in lung fibroblasts, in old mice and that bleomycin challenge further increased miR-34a and induced senescence and apoptosis in AECs⁴⁷. Ablation of miR-34a attenuated AEC senescence and lung fibrosis in old mice⁴⁷. Together, our data suggest that miR-34a expression may be distinctively regulated in different types of lung cells at different ages. In young mice, it is mainly expressed in fibroblasts and promotes fibroblast senescence and apoptosis and therefore protects mice from lung fibrosis. In old mice, however, it is mainly expressed in AECs, which is associated with AEC senescence and apoptosis as well as lung fibrosis. It should be stressed, although miR-34a associated fibroblast senescence seems to be beneficial in young mice in our study, this phenotype has been frequently implicated in the pathogenesis of lung fibrosis in aged animals and in patients. A plausible explanation for such a discrete effect may be attributable to relative contributions of different types of lung cells to this pathology between young and old subjects. Moreover, we want to point out that senescent fibroblasts are generally believed to be more resistant to cell death. However, this is not the case in miR-34a-induced phenotype change in lung fibroblasts. This suggests that other miR-34a induced effects may render these cells more conducive to apoptosis, which worth further investigations.

Senescent Cells as Therapeutic Targets for the Treatment of IPF

Senescent cells not only lose normal functions and regeneration capacity, they also secrete an array of pathogenically active molecules, which are detrimental to the surrounding tissue/cells. Therefore, removal of senescence cells has been proposed for the treatment of various aging-related diseases, including Alzheimer’s disease, diabetic kidney disease, and IPF^{13, 14, 16–18, 30}. Pan et al. reported that exposure of C57BL/6 mice to a single dose (17 Gy) of ionizing radiation induces a persistent pulmonary fibrosis 16 weeks after radiation exposure. Treatment of mice with a senolytic drug ABT-263 for 2 cycles (5 days/cycle) 16 weeks after ionizing radiation, on the other hand, significantly reduced senescent cells and reversed lung fibrosis¹³. Using the Ink-Attac mouse model, in which AP20187 induces apoptosis of p16 expressing cells, Schafer et al. showed that clearance of senescent cells (p16 expressing cells) significantly reduced bleomycin-induced inflammatory responses and lung fibrosis and improved lung function¹⁴. Treatment with dasatinib plus quercetin (DQ), a senolytic cocktail, replicated the beneficial effects of genetic depletion of p16 positive cells¹⁴. Similar results have been reported by another study from a different group, which confirmed the protective effect of DQ on bleomycin-induced lung fibrosis³⁰. Importantly, first-in-human open-label pilot study showed that administration of DQ to IPF patients, 3 times/week for 3 weeks, was tolerable and side effects were mild-moderate¹⁸. Most importantly, administration of DQ significantly improved the clinical measures, such as 6-min walk distance, and performance of IPF patients, although pulmonary function, clinical chemistries, and frailty index were unchanged¹⁸. This study provides initial evidence that senolytic drugs may have therapeutic potential for IPF.

In summary, cellular senescence has emerged as an important mechanism underlying the pathogenesis of IPF. More studies are needed to improve our understanding of the molecular mechanisms leading to cellular senescence in IPF lungs and the mechanisms whereby senescent cells contribute to the development of lung fibrosis. Most importantly, the efficacy of senotherapy for the treatment of IPF warrants further investigation.

Highlight.

1. Cellular senescence is a pathological feature of IPF lung and is evident in experimental lung fibrosis models.

2. Various types of lung cells, including alveolar epithelial cells, fibroblasts, and endothelial cells undergo senescence in fibrotic lung diseases

3. Different mechanisms are involved in lung cell senescence, including telomere shortening, epigenetic modifications, increased oxidative stress or inflammation, which leads to increased expression of senescence mediators PAI-1 and p53, as well as increased microRNAs, especially miR-34a.

4. Targeting senescent cells may be effective for the treatment of fibrotic lung disease.