Role of Inflammation and Oxidative Stress in the Pathology of Ageing in COPD: Potential Therapeutic Interventions

ABSTRACT

Ageing is defined as a progressive decline of homeostasis that occurs after the reproductive phase of life is complete, which is thought to arise because of impaired DNA repair following damage. This leads to an increased risk of disease or death. Ageing is one of the most important risk factors for most chronic diseases. Chronic obstructive pulmonary disease (COPD) represents an important component of the increasingly prevalent multiple chronic debilitating diseases that are a major cause of morbidity and mortality, particularly in the elderly. There is increasing evidence that the pathogenesis of COPD is linked to an accelerated ageing process. This review discusses the evidence supporting a number of mechanisms, including oxidative stress and ageing, in the pathogenesis of COPD. Greater understanding of these mechanisms leads to novel therapeutic interventions targeted at this heterogeneous disease.

KEYWORDS:

• Chronic obstructive pulmonary disease
• inflammation
• novel therapies
• oxidative stress
• senescence

Introduction

Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality worldwide. It has been projected to increase from the sixth to the third most common cause of death worldwide by 2020 and to rise from fourth to third in terms of morbidity within the same time frame (1). The prevalence of COPD is estimated to be around 1% of the adult population, but rises sharply amongst those aged 40 years and older, and continues to increase with increasing age (2).

Ageing is a multifarious, universal condition leading to the functional deterioration of all cells and organisms, and is an important factor in global health problems. Ageing and its associated cellular senescence are thought to be important as a pathogenic mechanism in COPD (3). The underlying mechanisms of ageing are still not fully understood. However, several mechanisms such as oxidative stress, telomere length regulation, cellular and immunosenescence, and changes in a number of anti-ageing molecules and in the extracellular matrix have all been implicated as key factors in this process (4–5).

This review article discusses these ageing mechanisms as they relate to the pathogenesis of COPD (Table 1) and potential therapeutic strategies targeted at these mechanisms that may provide novel therapies in this disease.

Table 1. Factors related to ageing that are potentially involved in the pathogenesis of COPD.

Mechanism	Pathogenic contribution in COPD
Oxidative stress	Circulating inflammatory cells produce reactive oxygen species (ROS) promoting injury and inflammation (6, 7)
Telomere length (TL)	Shortened telomere length is associated with reduced lung function emphysema and an increased risk of COPD (8)
Cellular senescence	P16/p21/s-β gal is increased in COPD lungs (9)
Anti-ageing molecules	Reduced level of SIRT1 and HDAC in lungs of COPD patients (10, 11)
Circulating cells	In COPD patients, neutrophils show enhanced ROS production (12), monocytes have defective phagocytosis, increase matrix metalloproteinase (MMP)-9 production (12)
	Lymphocytes: peripheral blood T cells show increased apoptosis and senescent T-cell phenotype (12, 13)
	NK cells: reduction of cytotoxic and phagocytic function (12–14)
Cytokines	Persistent high levels of systemic and pulmonary IL6, TNF-α and acute-phase reactants such as CRP, fibrinogen and surfactant protein D seen in COPD (5,12)
Advanced glycation end product (AGE)	COPD patients have lower levels of circulating
	AGEs correlating with the presence of emphysema (5,15)
Protease–antiprotease imbalance	Leads to lung parenchymal destruction (16)

Cellular senescence

Cellular senescence is defined as complete and irreversible loss of the replicative capacity in primary somatic cells (17). Normal human diploid fibroblasts can divide 50–70 times in tissue culture before they stop dividing (17). This type of growth arrest in which progressive telomere shortening leads to senescence is known as replicative senescence (18). Cellular senescence can also be a result of a response to stress, particularly oxidative stress-induced DNA damage – leading to stress-induced premature senescence (19).

Cellular senescence is associated with stereotyped phenotypic changes in cells characterised by (1) distinct, flat and enlarged cell morphology; (2) resistance to apoptosis; (3) development of a ‘senescence-associated secretory phenotype’ (SASP) with increased gene expression and production of inflammatory and growth mediators; and (4) an increase in senescence-associated galactosidase (SA βgal) activity (20–22).

At the molecular level, cellular senescence may be induced through either or both of the two pathways, the ARF-p53 pathway and the p16-retinoblastoma protein (pRb) pathway (23). The ARF-p53 pathway is activated by DNA damage, dysfunctional telomeres and genotoxic stresses. The p16-Rb pathway is also linked to cell cycle arrest in response to DNA damage and can be activated by oncogenes or other stresses.

During the normal cell cycle, the cyclin-dependant kinase (CDK) inhibitor p16INK4a seems to serve as a constant braking mechanism. Prior to senescence, cells exhibit an increase in p21Cip1/Waf1 expression, another important CDK inhibitor. This is controlled by the tumour suppressor and transcription factor p53, which plays a major role in the induction of cellular senescence. When cells become senescent, p21Cip1/Waf1 expression decreases, while that of p16INK4a increases. In accordance with this, p16INK4a and its pathway are considered primarily responsible for the final irreversible proliferation stop. Thus, p21Cip1/Waf1 can initiate senescence that is primarily dependent on telomere, which is then maintained and established by p16INK4a. Additionally, pRb is involved in the control of genes responsible for cell cycle progression and other functions by recruitment of the histone deacetylase (HDAC) 1 and HDAC complexes. This is important as understanding these mechanisms could potentially help us to identify therapeutic targets, as growing evidence now suggests that histone acetylation plays a critical role in the regulation of inflammatory genes and in mediating the anti-inflammatory effects, particularly that of corticosteroids in COPD patients (24). Therapeutic approaches to enhance HDAC activity may, therefore, help reduce chronic inflammation and the consequent ageing as seen in COPD patients.

When induction of senescence is based on DNA damage, senescent cells display a unique phenotype, which has been termed ‘senescence-associated secretory phenotype’ (SASP), and this has been^{Table 1} postulated to add further to the cascade of chronic inflammation as seen in COPD (25). This will be further discussed later on.

In one study, Amsellum et al. showed that higher percentage of senescent pulmonary endothelial cells (P-EC) stained for p16 and p21 in patients with COPD than in control subjects (26). P-ECs from COPD patients exhibited higher markers of replicative senescence, such as β-galactosidase activity, as well as a higher percentage of expressed inflammatory cytokine levels (IL-6, IL-8, soluble intercellular adhesion molecule, etc.) in the COPD patients. This again reinforces that senescence and chronic inflammation are integral factors and are interlinked in the pathogenesis of COPD.

Oxidative stress

Oxidative stress reflects an imbalance between the manifestation of oxidants and the ability of a biological system to detoxify these reactive oxidants or to repair the resulting damage. There is good evidence that such an imbalance between oxidants and antioxidants occurs in patients with COPD (27).

Several studies have shown that the levels of biomarkers reflecting oxidative stress are increased in COPD patients (28). Increased levels of 8-hydroxydeoxyguansoine, a marker of oxidative DNA damage, are found in urine and in the peripheral lung of smokers (with and without COPD) compared with non-smokers (29–30). Raised levels of 3-nitrotyrosine, a marker of nitrosative stress, and the lipid peroxidation product, F2α isoprostanes, were detected in the lungs of COPD patients, and these markers showed a strong correlation with the severity of airflow limitation as measured by forced expiratory volume in the first second (FEV¹) in COPD patients (25, 26). Increased levels of markers of oxidative stress are also found systemically in COPD patients (31–33). Thus, increased oxidative stress is a feature in COPD lungs, which occurs systemically in COPD patients and could be a major driver in the pathogenesis of the disease (Figure 1).

Figure 1. The role of oxidative stress in the pathogenesis of COPD (reproduced with permission from 33).

Oxidants

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) in COPD result from both cellular and environmental sources. Environmental oxidants arise predominantly from cigarette smoke, which contains 10¹⁵–10¹⁷ oxidants/free radicals and approximately 4,700 different chemical compounds, including reactive aldehydes and quinones per puff, resulting in an enormous oxidative burden in the lungs (28).

The ROS in cigarette smoke include non-radical species, such as hydrogen peroxide or oxygen radicals, superoxide anion (O₂⁻) and the hydroxyl radical, that are exceedingly unstable species with unpaired electrons capable of perpetrating further oxidation. Together with RNS, these can result in a variety of consequences such as the production of other ROS, lipid peroxidation, apoptosis, autophagy and consequent pulmonary inflammation (33, 34).

ROS and RNS can also be generated by inflammatory cells that influx into the lungs from structural lung cells (28). Inflammatory cells are activated once they enter the airspace and generate ROS in response to an appropriate stimulus (28). Leucocytes from smokers have been shown to release increased amounts of oxidants such as O₂− and H₂O₂ compared with those from non-smokers (35). The principal enzymes implicated in this process include nicotinamide adenine dinucleotide (reduced form) (NADH) oxidase as well as xanthine oxidase system and the haem peroxidases (27). RNS in the form of nitric oxide (NO) is generated by nitric oxide synthase, which adds to the oxidative stress. These oxidants can also stimulate the release of proteases leading to proteolysis of the matrix components and interfere with elastin synthesis and repair, and thus contribute to the development of emphysema (36).

In one study, Lanzetti et al. looked at the role of oxidative stress in elastase-induced pulmonary emphysema (37). C57BL/6 mice were subjected to pancreatic porcine elastase (PPE) instillation (0.05 or 0.5 U per mouse, i.t.) to induce emphysema. Also, mice treated with 1% aminoguanidine (AMG) and inducible NO synthase (iNOS) knockout mice received 0.5 U PPE (i.t.), and their lungs were analysed after 21 days. The study showed demonstrable emphysema as early as 21 days after receiving treatment in this group with raised levels of markers of oxidative stress, such as tumour necrosis factor (TNF)-α, myeloperoxidase and glutathione peroxidase, in all the groups treated with PPE. The study also showed that emphysema was attenuated when iNOS was inhibited using 1% AMG and in iNOS knockout mice, overall suggesting that the oxidative and nitrosative stress pathways are triggered by nitric oxide production via iNOS expression in pulmonary emphysema.

The mitochondria produce energy (ATP) through oxidative phosphorylation. However, in this process, a small percentage of electrons may ‘leak’ during normal respiration and prematurely reduce oxygen, forming ROS (38). As cells age, the function of the mitochondria tends to diminish, thus increasing electron leakage that, in turn, increases the production of ROS that can interact with lipid, proteins and nucleic acid, thus damaging vital cell components, including further damage to mitochondria. This is the basis of the free radical theory of ageing that focuses on the mitochondria as an increasing source of free radicals with ageing that leads to a vicious cycle where accelerated ageing then happens from accumulated damage inflicted by ROS (39).

Free radicals can accelerate replicative senescence via the shortening of telomeres, activate inflammatory redox-sensitive transcription factors such as nuclear factor κB (NFκB) and activator protein 1, which regulate the transcription of several genes encoding pro-inflammatory cytokines, and can induce DNA damage (32) adding further to the burden of oxidants.

Antioxidants

Normal healthy lungs, however, also have well-developed endogenous antioxidant defences that protect the lungs from the deleterious effects of these oxidants (40, 41). The fluid lining the respiratory tract forms an interface between the epithelial cells and the exterior and contains a high concentration of the antioxidant glutathione and so provides the first line of defence against inhaled oxidants. Nuclear factor erythroid 2-related factor (Nrf2) is a transcription factor that regulates phase II and antioxidant genes in response to oxidative stress. Studies have shown that decreased Nrf2 expression resulted in enhanced susceptibility to cigarette smoke- and elastase-induced emphysema in mice, associated with more significant oxidative stress in lungs (42–44). Studies have also shown that NRF2-dependent antioxidants were negatively associated with severity of COPD. Therapy directed towards enhancing NRF2-regulated antioxidants could be a novel strategy for attenuating the effects of oxidative stress in the pathogenesis of COPD (45).

Activation of the unfolded protein response

The unfolded protein response (UPR) encompasses a series of transcriptional, translational and post-translational progressions that reduces protein synthesis while enhancing protein-folding capacity and protein degradation. Because of the potentially important role of UPR as a quality control process in protein metabolism, it likely plays an important role in the pathogenesis of several chronic conditions.

Recent studies suggest a link between the activation of the UPR and emphysema. Cells exposed to cigarette smoke develop endoplasmic reticulum (ER) stress, which induces a compensatory response (UPR) (46, 47). This response alters the expression of a variety of genes involved in antioxidant defence, inflammation, energy metabolism, protein synthesis, apoptosis and cell cycle regulation. The proteomes of lung samples from chronic cigarette smokers demonstrated 26 differentially expressed proteins compared with non-smokers (47). Amongst the upregulated proteins, several were involved in the UPR as well as enzymes involved in antioxidant defence. This upregulation of UPR regulatory proteins by cigarette smoke may provide a pro-survival phenotype by increasing resistance to cytotoxic stresses such as hypoxia and therefore may have a protective role in the lung against oxidant injury and consequently slow down ageing in COPD (48). Heightened UPR may, however, contribute to lung cell apoptosis; thus, the role of UPR as a protective or a contributory factor in COPD remains a topic of debate (49).

Role of transcriptomic and lipidomics in oxidative stress burden

Heightened ER stress in COPD has been ascribed mechanistically to decrease in proteosomal activity and proteosome gene expression (49). The reductions in proteosomal activity were explained in part by decreased expression of Nrf2, which stimulates expression of key components of the 20 S proteosome in the presence of oxidative stress. Decreased Nrf2 expression and decreased expression of key Nrf2/ATF4-regulated antioxidant enzymes such as heme-oxygenase-1 have also been reported in COPD, suggesting that these signalling pathways may also have an implication on antioxidant gene expression in COPD (50).

The role of lipids in cell, tissue and organ physiology has been demonstrated by a large number of genetic studies that involve the disruption of lipid metabolic enzymes and pathway and therefore may have a role to play in the pathogenesis of chronic lung condition such as COPD (51). Telenga et al. studied lipid expression in induced sputum of COPD patients (smokers and ex-smokers) and compared them with non-smokers (52). They also looked at the changes in lipid expression in the smoking COPD cohort after 2 months of smoking cessation. They showed that there were more than 1500 lipid compounds identified in the sputum samples. The class sphingolipids were found in significantly higher levels in smokers with COPD than in smokers without COPD. There were 13 lipid compounds including sphingolipids that had significantly higher expression in the smoking COPD group compared with the ex-smokers. After two months of smoking cessation, there was a reduction in the expression of 26 sphingolipids in smokers with and without COPD. This study shows that identifying the lipidomic pathway may give further insight into the pathogenesis of COPD and may help us identify new drug targets and potential biomarkers in COPD patients. Similarly, the α,β-unsaturated aldehyde 4-hydroxy-2-nonenal (4-HNE), a specific lipid peroxidation product, has been found at increased levels in the lungs of patients with COPD as well as in the animal models of this lung disorder (53).

Increased lung cell autophagy, apoptosis, and efferocytosis: Role of sirtuins

Autophagy is an adaptive mechanism to protect cells from stress-induced injury, which removes superfluous and damaged mitochondrial organelles (54). It is an evolutionary-preserved process involved in the degradation of long-lived proteins, which becomes apparent during states of increased stress and therefore may be considered as a cell survival mechanism (55, 56). Sirtuins are type III histone deacetylases (HDAC) that act on histone residues in DNA. Histone deacetylase 2 (HDAC2) is a class-I histone deacetylase that regulates various cellular processes, such as cell cycle differentiation, apoptosis, autophagy and senescence (57, 58). Within the HDACs, sirtuins maintain a special position as they are structurally different from other HDACs, are inhibited by different compounds and have the unique characteristic of being nicotinamide adenine dinucleotide (NAD)+ dependent (59). Amongst the sirtuins, sirtuin1 (Sirt1) is important in the context of COPD and is thought to be involved in the clearance of old and damaged mitochondria by prompting autophagy. It also aids in the activation of the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-α), the mitochondrial biogenesis regulator and thereby facilitates the recruitment of new mitochondria (60). Given the central role of mitochondria in the ageing process (61, 62), activation of Sirt1 by maintaining and potentially rejuvenating the pool of mitochondria is a potential therapeutic option. In this respect, resveratrol (3, 4′, 5-trihydroxystilbene), a plant polyphenol, deserves mention (62). Resveratrol appears to facilitate its anti-ageing effects partly by stimulating SIRT1 and is also reported to be a powerful antioxidant as well as a PI3K inhibitor. This will be discussed in more detail later in this article.

Although principally characterised as a cell survival mechanism, the relationship between autophagy and cell death pathways is only partly understood, as excessive autophagic activity may actually cause the death of critical cell populations and thereby hasten disease processes (63). In this context, Chen et al. have shown increased autophagy in bronchial epithelial cells from patients with COPD compared with those from healthy individuals, as assessed by morphological and biochemical markers such as increased expression and activation of autophagic regulator proteins (such as Light Chain 3B (LC3B), Beclin 1, Atg5 and Atg7) as well as electron microscopic evidence (64). In addition, pulmonary epithelial cells exposed to cigarette smoke extract (CSE) decreased HDAC activity, resulting in increased binding of early growth response-1 (Egr-1) and E2F factors to the autophagy gene LC3B promoter and augmented LC3B expression, thus promoting autophagic cell death. Furthermore, knockdown of Egr-1 inhibited the expression of Atg4B, a critical factor for LC3B conversion and inhibition of autophagy by LC3B-knockdown, and protected epithelial cells from CSE-induced apoptosis. These results suggest that modulation of autophagic pathway may have a potential therapeutic option in the management of COPD.

Apoptosis (programmed cell death) is a critical process for the maintenance of normal tissue homeostasis and is equipoised with proliferation and differentiation in the cell cycle homeostasis. Increased apoptosis has been shown in epithelial and endothelial cells as well as in inflammatory cells including neutrophils and lymphocytes in the lungs of COPD patients (65). It has been postulated that apoptosis can add to the burden of the oxidative stress. Cell death and tissue trauma as triggered by apoptosis can cause release of self-antigens, impairment of mitochondrial function, protein alterations and the release of truncated DNA from the apoptotic cells (66, 67). These products in turn can trigger a foreign body-like reaction when recognised by the adaptive immune system adding additional oxidative stress load.

Efferocytosis is a process that allows for the removal of apoptotic material with minimal inflammation and averts the development of secondary necrosis and ongoing inflammation. Defective efferocytosis and consequent increased number of apoptotic cells have been identified in the airways of subjects with COPD (68). Apoptotic cells therefore accumulate in COPD patients not only because of increased apoptosis but also because of defective recognition of apoptotic cells by apoptotic mediators and impaired efferocytosis (69). In fact, Etboli et al. have shown that macrophage efferocytosis of eosinophils is impaired in COPD and is probably related to the severity and frequency of COPD exacerbations (70). It is also now known that impaired efferocytic clearance of apoptotic epithelial cells by alveolar macrophages occurs in COPD, cigarette smoking and other lung inflammatory diseases and might be related to impaired zinc homeostasis in macrophages of COPD patients (71). Hodge et al. studied the possible role of azithromycin-mediated improvement in efferocytosis by a mechanism involving collectins (mannose-binding lectin and surfactant protein (SP)-D) and mannose receptor (MR) in a group of COPD patients (72). Azithromycin was administered to 11 COPD patients. The study showed a remarkable improvement in AM phagocytic ability (pre: 9.9%; post: 15.1%), reduced bronchial epithelial cell apoptosis (pre: 30.0%; post: 19.7%), and increased MR and reduced inflammatory markers in the peripheral blood after azithromycin therapy, indicating a potential role of the antibiotic in improving efferocytosis in this cohort. This could therefore be a potential novel therapy in COPD in the future.

Telomere length shortening in COPD

Telomeres are repetitive DNA sequences (TTAGGG/CCCTAA) located at the end of the chromosomes, protecting chromosomes against degradation and remodelling (73). Due to the end-replication problem in mature somatic cells, telomere repeats are lost with each replicative cycle, until a critical length is attained when the cells stop replicating and undergo apoptosis or senescence. Inflammation or oxidative stress can amplify this process significantly leading to shortened telomere lengths (74, 75). Assessing telomere length (TL) is thus a marker of biological age. In the Lung Health Study (76), a cohort of 5887 smokers aged 35–60 years with mild-to-moderate airflow limitation, TL in peripheral blood leucocytes of patients with COPD was significantly related to the risk of all-cause mortality (Hazard Ratio, HR, 1.29; p  =  0.0425) over a median follow-up of 7.5 years, irrespective of potential confounders such as chronological age, smoking status and lung function (77).

In another observational study, Rode et al. studied almost 10,000 patients with COPD and showed that telomere length in blood leucocytes decreased significantly with increasing age and with decreased FEV¹: FVC ratio, indicating a relationship between TL and the degree of airflow limitation (15). Similar results were also shown in another cohort of 283 COPD patients (78). Interestingly, Debigaire et al. has recently shown shortened TL from the quadriceps muscle biopsy of COPD patients compared with healthy subjects, suggesting a role for ageing in the pathogenesis of the muscle dysfunction in COPD (79).

Animal studies have also shown that animals with shorter telomeres in their lung cells have increased susceptibility to cigarette smoke-induced emphysema (80). Studies have also shown that in circulating leucocytes, both current and ex-smokers had shorter telomeres than their age-matched non-smokers with a dose-dependent relationship between TL and the years smoked, and TL in patients with COPD is shorter than that of control subjects in any age range (81).

Parenchymal lung cells from emphysematous lungs also show shortened telomeres, and this is associated with increased cell senescence (82). These events may further enhance lung inflammation and consequent enhanced ageing as seen in COPD patients. Table 2 summarises the possible mechanisms of TL shortening in COPD.

Table 2. Possible mechanisms of telomere length shortening in COPD.

Oxidative stress from exogenous and endogenous sources (31–32).Chronic inflammation as observed in COPD (76).Genetic predisposition of COPD patients to accelerated telomere shortening (78)Exercise can enhance the production of reactive oxygen species, such as superoxide anions, hydrogen peroxide and hydroxyl radicals, thus increasing the rate of oxidative damage to DNA particularly in COPD muscles (80)

Inflammation in COPD: Role in ageing

Chronic lung inflammation is a characteristic feature of COPD and involves cells such as neutrophils, macrophages, T lymphocytes and augmented concentrations of leukotriene B4, interleukin (IL)-1, IL-6 and IL-8b, and TNF-α (83). Bronchoalveolar lavage fluid and sputum have previously demonstrated increased inflammatory biomarkers such as cytokines, proteases and soluble cytokine receptors in COPD patients (84).

In addition to local lung inflammation, many studies have shown increased systemic levels of inflammatory mediators in COPD, such as IL-6, TNF-α, IL-8/CXCL8 and C-reactive protein (CRP) (85). Agusti et al. defined systemic inflammation as having two or more elevated blood inflammatory biomarkers – IL-6, white blood cell count, high-sensitivity CRP and fibrinogen persistently elevated, and found persistent systemic inflammation in 16% of COPD patients, which was associated with worse clinical outcomes including a six times increased mortality (85).

Also low-grade systemic inflammation is thought to be a common biological factor responsible for the decline and the onset of disease in the elderly, the so-called inflamm-ageing hypothesis, and might also add to the ageing burden of COPD patients (86). This is important as control inflamm-ageing may be a strategy to abrogate the major age-related pathologies. It is therefore important to clarify the complex mechanisms of inflamm-ageing in order to carry out targeted therapeutic interventions in the elderly population (87).

Information from parenchymal and bronchial biopsies of COPD patients have provided us with further insight in the role of inflammatory pathways in the pathogenesis of COPD patients (88). Bronchial biopsies from patients with COPD had shown predominance of CD4+, CD8+ cells and macrophages expressing nuclear factor-kappa B (NF-kappaB), STAT-4 and IFN-gamma proteins as well as endothelial adhesion molecule-1 in endothelium in mild/moderate disease, whereas the severe end of the disease spectrum has a preponderance of total and activated neutrophils (myeloperoxidase [MPO]+ cells) along with increased immunoreactivity (84). Upregulation of pro-inflammatory transcription factors NF-kappaB and STAT-4 in mild disease and that of activated epithelial and endothelial cells in the more severe disease may explain this difference in the prevalence of inflammatory cells in different stages of the disease. Studies have also shown that the DNA damage of lung epithelial barrier cells due to oxidative stress leads to increased expression of 8-hydroxydeoxyguanosine (8-OHdG). This is an oxidised nucleoside of DNA that leads to further generation of ROS, diminished DNA auto-repair ability and ultimately somatic mutations leading to persistence of oxidative stress and somatic mutations, even after smoking cessation, thus promulgating further inflammation (89, 90).

Role of NF-κB pathways in lung inflammatory response

Amongst the many transcription factors involved in the regulation of the inflammatory proteins, nuclear factor (NF)-κB deserves special mention as it plays a key role in the expression of many pro-inflammatory genes, leading to the increased expression of cytokines, adhesion molecules, chemokines, growth factors and enzymes (91). NF-κB repressing factor (NRF) is a transcriptional repressor that is widely expressed in many human tissues and has been implicated in the basal silencing of specific NF-κB-targeted genes, including inflammatory cytokines such as interferon-β, IL-8/CXCL8 and inducible nitric oxide synthase (91, 92). In short, the expression of most of the genes for the pro-inflammatory mediators involved in inflammatory process in the lungs in COPD, including IL-1, IL-6, IL-8, MCP-1 and TNF-α, are all activated by NF-κB. While NF-κB is required for cell survival and immunity, abnormal expression and/or activation of NF-κB contributes to the development of many pathological conditions, particularly chronic inflammatory conditions such as COPD. Studies have shown that there is enhanced NF-κB activation in the lungs of COPD patients (93, 94).

Activation of NF-κB depends upon the relative levels of NF-κB and its inhibitor proteins. Under normal conditions, all NF-κB is retained within the cytoplasm linked to its inhibitor IκB. Augmented expression of NF-κB, particularly the p65 subunit, can overcome the inhibitory effects of IκB leading to nuclear translocation and increased inflammatory gene expression as seen in the airways of smokers and subjects with COPD (95, 96).

In addition, there is increased nuclear expression of NF-κB (p65) in bronchial biopsies of COPD patients (94). Likewise, the numbers of RelA/p65-positive and RelA/p65 nuclear expression in epithelial cells and macrophages are increased in patients with COPD that correlate with the severity of the airflow limitation (91). Nuclear NF-κB activation has also been shown to be increased in senescent alveolar-type II cells from COPD patients compared with healthy smokers and non-smoking controls, (97). Additionally, compared with non-smoking controls, there is decreased SIRT1 protein expression that negatively regulates transcription factors, such as NF-κB, in the lungs of COPD patients (98).

Several different signal transduction pathways and a wide variety of cellular stresses and stimuli are known to activate NF-κB, including physical and chemical stress, lipopolysaccharide (LPS), dsRNA, ssRNA, T and B cell mitogens and pro-inflammatory cytokines (95, 99). These converge on a single target, the NF-κB /IkappaB complex and its activating kinase (inhibitor of kappaB kinase, IKK). The final activation occurs through two main pathways: the canonical and the alternative pathways. This is of therapeutic importance as inhibitors of NF-κB signalling pathways may be a novel anti-inflammatory strategy in COPD with implications for inflammation and oxidative stress-induced ageing in COPD.

Figure 2. Interaction between oxidative stress, inflammation and protease-antiprotease balance in senescence and pathogenesis of COPD.

Role of senescence-associated secretory phenotype

Role of cellular senescence in the pathogenesis of COPD has already been discussed before. When induction of senescence is based on DNA damage, senescent cells display a unique phenotype, which has been termed ‘senescence-associated secretory phenotype’ (SASP), and this has been postulated to add further to the cascade of chronic inflammation as seen in COPD (100). SASP factors are designed to signal immune cells for the removal of senescent cells. However, if this removal process is impaired or if the number of senescent cells in a tissue is too high, as is observed sometimes in COPD, the senescent cells might persist and maintain the secretory phenotype, adding to the burden of inflammation and ageing. Stress-induced senescence via an intense DNA damage leads to the DDR that has been postulated responsible for this SASP response through a multitude of inflammatory cytokines such as IL6 and IL8. This relationship between SASP and pathogenesis of COPD needs further exploration.

Advanced glycation end products

Advanced glycosylation end products of proteins are non-enzymatically glycosylated proteins, which accumulate in vascular tissue with age (101). They have a broad range of cellular functions, particularly in endothelial cells and macrophages, such as expression of pro-coagulant activity, inducing migration of mononuclear phagocytes, as well as production of platelet-derived growth factor and cytokines (102, 103) that add to the inflammatory cascade.

The receptor for advanced glycation end products (RAGEs) is a multi-ligand signal transduction receptor that can initiate and propagate inflammation induced by a variety of stimuli, including hyperglycaemia, oxidative stress, ageing and hypoxia (22, 102). RAGE has several isoforms including a soluble form known as sRAGE that appears to act as a decoy receptor, binding RAGE ligands in the extracellular fluid and thus offering protection against inflammation and oxidative stress (102) and so has the potential to prevent the some of the pathogenic manifestations of COPD.

A decline in some of the RAGE isoforms has been shown in COPD patients, further suggesting that it is involved in the pathogenic mechanisms in COPD (104). A study by Rutten et al. have also shown that sRAGE has a significant and independent association with FEV¹, FEV¹/VC and diffusing capacity of the lung for carbon monoxide (DLCO) in COPD patients, suggesting that it may be a marker of disease severity and consequently could be a marker of accelerated ageing in this cohort (105). Miniati and co-workers in another study involving 200 COPD patients and 201 age- and sex-matched controls showed that the reduction of sRAGE in COPD was strongly associated with the impaired DLCO, and the severity of emphysema as measured by CT (22).

High-mobility group protein B1 (HMGB1) is an abundant chromatin protein that acts as a cytokine when released into the extracellular milieu by necrotic and inflammatory cells. It is therefore considered a marker of tissue injury and a mediator of inflammation (106). Recent studies have reported an enhanced level of HMGB1 in bronchoalveolar lavage (BAL) fluid, sputum and circulation in patients with COPD (101). sRAGE presumably inhibits the activity of HMGB1 and therefore might be of potential therapeutic importance in interrupting the inflammatory cascade that contributes to accelerated ageing in COPD.

Role of mitochondrial dysfunction in ageing in COPD

Mitochondria are organelles that are an integral part in regulating metabolism, cell proliferation, energy production and survival. Environmental stress can lead to mitochondrial dysfunction and alterations may include reduced and abnormal mitochondrial biogenesis, oxidative phosphorylation and ATP production. This may lead to augmented generation of ROS, inflammatory responses and accelerated cellular senescence (107). Ageing has indeed been linked to mutations of the mitochondrial gene, which encode key proteins of the respiratory chain complex (108, 109). Chronic oxidative stress may prompt mitochondrial DNA (mtDNA) damage and subsequent accumulation of variant mtDNA sequences that can result in abnormal oxidative phosphorylation causing increased cytotoxicity, apoptosis and accelerated senescence (104). Damaged mitochondria may not be properly cleared by mitophagy, and accumulation of damaged mitochondria can have an effect on mitochondrial biogenesis by generating additional ROS and mtDNA mutations. Other than by producing increased amount of ROS, evidence also suggests that mitochondrial metabolism is also important in mediating longevity through nutrient-sensing pathway involving insulin/IGF-1 and mTOR signalling pathways (104). Enhanced ROS may further activate this signalling pathway leading to cell cytotoxicity, apoptosis, senescence and stem cell exhaustion. Studies have shown that qualitative alterations in skeletal muscle mitochondrial respiration, associated with reduced mitochondrial efficacy, may contribute to impaired exercise tolerance in patients with COPD. In one such study, Gifford et al. looked at permeabilised muscle fibres from the vastus lateralis of 13 patients with COPD and 12 healthy controls (110). They measured complex I (CI) and complex II (CII)-driven State 3 mitochondrial respiration. To ascertain whether an altered pattern of respiration represented qualitative changes in mitochondrial function, respiration states were examined as percentages of peak respiration, which revealed altered contributions from State 3:CI (Con 83.7 ± 3.4, COPD 72.1 ± 2.4%Peak, p < 0.05) and State 3:CII (Con 64.9 ± 3.2, COPD 79.5 ± 3.0%Peak, p < 0.05) respiration, reinforcing the fact that alterations in skeletal muscle mitochondrial respiration potentially can lead to exercise intolerance and accelerated physiological senescence as associated with this disease.

Protease–antiprotease imbalance in COPD

A pathogenic triad of inflammation, protease–antiprotease imbalance and oxidative stress has been considered as key mechanisms in the pathogenesis of COPD (40). This mechanistic triad is thought to be responsible for the mucous/goblet cell metaplasia and hyperplasia, mucous hypersecretion, fibrosis, smooth-muscle alterations and lung tissue destruction that is observed in COPD (41). The free radical theory of ageing also suggests that ROS, modifiable by genetic and environmental factors (e.g. smoking and pollutants), facilitate ageing -associated changes in cells and tissues (111).

Figure 2 summarises the interaction between oxidative stress, inflammation and protease–antiprotease balance in senescence and the pathogenesis of COPD.

A protease/antiprotease imbalance is thought to result from an increased release of proteases by inflammatory cells and from oxidative inactivation of protease inhibitors and is thought to be responsible for the lung parenchymal destruction in COPD (112–115). Smoke exposure from cigarettes or other biomass smoke and resultant oxidants can inactivate endogenous antiproteases, activate resident alveolar macrophages and promote neutrophil influx into lungs (113, 116). These activated leucocytes release proteases, such as neutrophil elastase (113, 117). Proteinases act on components of the extracellular matrix, elastin fibres and collagen and produce chemotactic peptide fragments that attract the further macrophage and neutrophil influx, initiating further inflammation, oxidative stress and consequent lung damage.

In one study, Overbeek et al. demonstrate the role of chemotactic collagen fragments such as N-acetylated proline–glycine–proline (N-ac-PGP) as an activator of Mac-1 on the surface of neutrophils as well an inducer of CD11b/CD18-dependent neutrophil adhesion to endothelium (118). Neutrophils were isolated from fresh human blood and a transmigration assay was then performed to evaluate the active migration of the cells towards N-ac-PGP. Results confirmed that neutrophils indeed transmigrate through an endothelial cell layer in response to N-ac-PGP, which also induced neutrophil adherence to fibrinogen. This could be another potent mechanism contributing to neutrophilic transmigration into the lung tissue during lung inflammation, as observed in COPD.

Therapeutic options: Potential novel interventions in COPD

Therapeutic options to combat the above-discussed pathogenic mechanisms in COPD are now of heightened interest. Various approaches have been tried to redress the oxidant–antioxidant imbalance in COPD patients (33). The use of antioxidants with good bioavailability or molecules that have antioxidant enzyme activity could not only protect against the deleterious effects of oxidants but may also alter the inflammatory chain of events that are thought to play a critical role in the pathogenesis of COPD.

Glutathione and N-Acetylcysteine

As discussed previously, alveolar epithelial cells are important in maintaining the integrity and fluid balance of the lung and in the control of inflammation. This is partly mediated by the antioxidant effect of glutathione (GSH) that is present in concentrations of 500 ∼M in the epithelial lining fluid, conferring protection against the detrimental effects of cigarette smoke (119). Extracellular and intracellular glutathione appear to be vital to the preservation of epithelial integrity following cigarette smoke exposure. Both in vitro and animal work has shown that smoke exposure causes the depletion of intracellular glutathione respiratory epithelial cells associated with increased epithelial permeability (120, 121). Previous in vivo and in vitro studies have shown that depletion of lung GSH alone, by treatment with the glutathione synthesis inhibitor buthionine sulphoxamine, can induce increased airspace epithelial permeability, indicating the protective role of GSH on the respiratory epithelial cells (121, 122).

Linden and co-workers have also shown that airflow limitation, as measured by FEV¹ in smokers with chronic bronchitis or COPD, correlated significantly with the concentration of GSH in bronchoalveolar lavage fluid (123).

In this context, N-acetylcysteine (NAC) is a thiol-containing compound that detoxifies reactive electrophiles and free radicals non-enzymatically, either through conjugation or reduction (109). Studies have shown that NAC, by regulation of the redox status in cells, can impede several signalling pathways that play a role in the regulation of apoptosis, angiogenesis, cell growth, nuclear transcription and cytokine production (124).

Some studies have shown that NAC has an anti-inflammatory effect. Van Schooten and co-workers (125) showed a decrease in neutrophil chemoattractant properties of the COPD patients' sputum after 10 months of NAC treatment at 600 mg/day. Another study has shown a beneficial effect of NAC on muscle function by demonstrating an increase in quadricep endurance time in severe COPD patients associated with a diminution in markers of systemic oxidative stress (126).

In the Pantheon study (127), the investigators enrolled patients aged 40–80 years with moderate-to-severe COPD in 34 hospitals in China. Patients were stratified according to the use of inhaled corticosteroids (regular use or not) at baseline and randomly allocated them to receive N-acetylcysteine (one 600 mg tablet, twice daily) or matched placebo for 1 year. The rate of exacerbation was significantly reduced in the group that received NAC compared with the placebo group after a year's follow-up (95% CI 0·67–0·90; p  =  0·0011). This suggests a potential role of NAC in oxidative stress and inflammation as observed in COPD.

However, the Bronchitis randomized on NAC cost-utility study (BRONCUS) of 523 patients with COPD, who were randomly assigned to 600 mg daily NAC or a placebo (128) and followed up for 3 years, showed that there was no significant difference between the two groups in the prevention of decline in FEV¹ or rate of exacerbations.

One reason for the lack of efficacy of NAC could relate to the fact that NAC has to be deacetylated in the gut to cysteine to act as a precursor of GSH and is therefore not very bioavailable to increase in GSH concentrations. Further studies may be warranted using NAC at higher doses (1200 or 1800 mg/day) or using other thiol agents that have a greater bioavailability in COPD patients.

Carbocysteine

Carbocysteine (S-carboxymethylcysteine) is a thiol derivative of the amino acid, l-cysteine, and has mucoactive, antioxidant and anti-inflammatory properties (129). Studies have shown that treatment of COPD patients with carbocysteine for six months significantly decreased the levels of the lipid peroxidation product, 8-isoprostane as well as levels of IL-6, signifying that the drug has both antioxidant and anti-inflammatory properties (130).

In the Peace Study, 709 COPD subjects in China were treated with carbocysteine 250 mg three times a day or placebo. The treated group experienced fewer numbers of exacerbations per year (131). This effect was present in a group of patients in whom the majority had not been treated with inhaled corticosteroids. Carbocysteine has also been shown to reduce the frequency of common colds and associated exacerbations in COPD patients, a property that has been attributed to its ability to decrease ICAM-1 expression in the respiratory tract (33). It therefore holds promise as an antioxidant and anti-inflammatory drug in COPD on top of the mucolytic property that is currently its predominant use in this cohort.

Ambroxol

Ambroxol is a secretolytic mucoactive agent and has been postulated in many studies as a potential antioxidant in COPD patients (132, 133). In one such study, Ricciardolo et al. assessed the additive effect of ambroxol and beclomethasone dipropionate in inhibiting LPS-induced expression of various inflammatory cytokines including IL-8, inducible NO synthase (iNOS), myeloperoxidase (MPO), etc., in human bronchial epithelial (BEAS-2B) and leucocytes treated with LPS (132). This study showed that LPS inhibited release of IL-8 and also inhibited iNOS expression. The synergistic effect of ambroxol and beclomethasone also inhibited LPS-stimulated IL-8, MPO and 3-NT release and therefore could have potential antioxidant and anti-nitrosative effects in neutrophil predominant chronic inflammatory conditions such as COPD. At least one study has shown an effect of ambroxol in diminishing exacerbations also in COPD patients (133).

Nrf2 activators

As discussed previously, nuclear factor erythroid 2 p45-related factor 2 (Nrf2) is a basic-leucine zipper (b-ZIP) transcription factor present in the cytoplasm of normal cells and plays a central defensive role against electrophiles and ROS (36). In response to oxidative and electrophilic stresses, Nrf2 detaches from its cytosolic inhibitory subunit, Kelch-like ECH-associated protein 1 (Keap1), and translocates into the nucleus where it binds to the antioxidant response element (ARE) of target genes (134, 135).

There is considerable interest in devising novel therapies that could be potent activators of Nrf2 or which stabilise Keap1 and DJ-1 (another stabiliser of Nrf2). In a mouse model, it has been shown that activating Nrf2 with the compound CDDO-imidazolide can diminish LPS-induced inflammation and mortality and hence could potentially provide a novel therapeutic strategy in COPD patients (136).

Combating steroid resistance in COPD

It has been postulated that oxidative stress plays a role in the poor efficacy of corticosteroid treatment in COPD patients. Ito et al. have shown a role for histone acetylation and deacetylation in IL-1β-induced TNF-α release in alveolar macrophages derived from cigarette smokers (137). They also proposed that TNF-α release was enhanced by the HDAC inhibitor Trichostatin A and correlated significantly with HDAC activity. Using a macrophage cell line, they also showed that hydrogen peroxide resembles the effects of cigarette smoke on HDAC activity and significantly attenuates dexamethasone inhibition of cytokine release. In addition, glucocorticoid responsiveness was reduced in alveolar cells and correlated with HDAC activity (in cell lines obtained from biopsy samples and BAL macrophages) of healthy smokers compared with age-matched non-smokers. This probably results from the fact that to suppress the inflammatory genes by glucocorticoids, there is a need for recruitment of histone deacetylase-2 (HDAC2) to the transcription activation complex by the glucocorticoid receptors (137, 138).

Therefore, a possible novel therapy in COPD would be to increase HDAC2 expression and activity so that steroids can recover their anti-inflammatory activity. NAC (139), theophylline (140) and upregulation of HDAC2 activity by polyphenols (141) have all been postulated as potential methods to achieve this.

Polyphenols

Polyphenols are secondary metabolites of plants and are of interest as potential antioxidants. Epidemiological studies and associated meta-analyses strongly suggest that long-term consumption of diets rich in plant polyphenols offer protection against development of cancers, cardiovascular diseases, diabetes, premature ageing, osteoporosis and neurodegenerative diseases (142).

Tabak and co-workers assessed dietary intake of polyphenols (catechins, flavonols and flavones) in relation to pulmonary function and COPD symptoms in 13,651 patients (143). Their study showed that single component such as catechin intake was independently associated with improvement in FEV¹ and all three COPD symptoms (cough, breathless and sputum production), while flavonol and flavone intake was independently associated with improved chronic cough only but not with FEV¹.

In another study, Santus and co-workers showed that treatment with a standardised polyphenol extract in COPD patients produced a significant decrease in urinary isoprostane excretion as well as an improvement in oxygenation (measured by PaO₂) and in FEV¹ between enrolment and the end of the study (144). Another polyphenol that deserves a mention in this context is curcumin (diferuloylmethane), which is a naturally occurring flavonoid present in the spice turmeric. Curcumin has also been reported to inhibit inflammation and acts as an effective antioxidant by inhibiting NF-κB activation and thus IL-8 release, cyclooxygenase-2 expression and neutrophil recruitment in the lungs (145). Polyphenols, particularly curcumin, could therefore help upregulating HDAC activity in macrophages and thereby could improve steroid resistance in COPD (141).

Spin traps and iNOS Inhibitors

Spin traps are chemical agents that can engulf free radicals to form measurable stable end products. They are established around a nitrone- or nitroxide-containing molecule and these have been widely used for in vitro studies and their therapeutic effects in vivo have also been investigated in models of lung inflammation (146). Surprisingly, no spin traps have been studied in COPD patients.

However, animal studies have suggested that inhibition of inducible nitric oxide synthase (iNOS) by various chemical inhibitors decreased emphysema in various animal models (147). It is therefore conceivable that selective inhibition of iNOS along with judicious use of other antioxidants may provide a therapeutic strategy in the management of COPD. A human study in COPD patients to assess the efficacy of these molecules would indeed be desirable.

Redox sensor inhibitors

Redox sensors, which can affect the overall redox state of the cell, include the NADP/NADPH and reduced glutathione (GSH)/oxidised glutathione (GSSG) systems. In this context, the oxidoreductase family of redox sensors deserve a special mention, the most notable amongst them being the thioredoxin (Trx) and redox effector factor-1 (Ref-1) (148). Trx has been shown to play a vital role in regulating redox-sensitive signalling pathways, such as NF-κB and AP-1, p38MAPK, and c-Jun N-terminal kinases (JNK) (149). Not surprisingly, inhibiting Trx (in the nucleus) with an inhibitor like MOL-294 blocks nuclear activation of both NF-κB- and AP-1-dependent transcription, resulting in diminished neutrophil influx and TNF-α production as shown in animal studies (149). Thus, upregulation of Trx by various synthetic small molecules could have therapeutic anti-inflammatory and antioxidant potential for the treatment of COPD.

Lipid peroxidation and protein carbonylation inhibitors/blockers

In this group, the lazaroids (U75412E or tirilazad mesylate) and edaravone (MC-186) are worthy of a mention. The MC-186 exhibits its antioxidant ability by inhibiting lipid peroxidation and protein carbonylation (150), whereas lazaroids are a group of non-glucocorticoid aminosteroids and can infiltrate the hydrophobic regions of the cell membrane, preventing peroxidation of the membrane lipids (151). However, both these compounds need further analyses before they could be targeted as potential therapeutic agents as antioxidants in COPD patients.

Enzymatic antioxidants

Cellular ROS can be successfully neutralised by antioxidant enzymes, such as superoxide dismutase (SOD), catalase and glutathione peroxidase. This is important as these enzymes could be impaired in COPD patients (152, 153). Restoration of these altered antioxidant enzyme activity by using enzyme mimetics could be a possible therapeutic option in COPD.

Superoxide dismutases

As discussed above, numerous studies have confirmed the protective role of SOD against oxidants in the lung. SOD is induced by cytokines such as TNF-α, oxidants and cigarette smoke. A number of SOD mimetics based around organo-manganese complexes have been developed, which retain their antioxidant properties in vivo. These include a series of ligands, such as M40401, M40403 and M40419 (first-class ligands) manganese–metalloporphyrins, such as AEOL-10113 and AEOL-10150 (second-class ligands) and manganese-based SOD mimetic ‘salens’ (third-class ligands) and have been devised for this purpose (152). The ‘salens’ are also known to have catalase-like activity and can scavenge ONOO⁻ and hydrogen peroxide radicals that are generated as a by-product of SOD activity, thus adding to its antioxidant property (153).

Animal studies have shown a substantial decrease in the markers of oxidative stress in the lungs and emphysema in response to the SOD mimetic M40419 (154). Animal studies have shown AEOL-10150 to impede CS-induced lung inflammation in rats with diminished lipid peroxidation and the generation of ONOO⁻ (152). In this context, recombinant SOD may be a promising future anti-inflammatory agent in COPD as it has also shown to prevent neutrophil influx into the lungs and decrease IL-8 release induced by cigarette smoking in animal models (155).

Extracellular SOD (ECSOD or SOD3) is highly expressed in lungs and is located in the extracellular matrix (ECM) (156). SOD3 acts in several ways including direct scavenging of free oxygen radicals (31), reduction of CS-induced oxidative stress in macrophages as shown in mouse models (157) as well as reducing lung inflammation and emphysema by lessening oxidative fragmentation of extracellular matrix components such as heparin sulphate and elastin (158). Therefore, SOD3 holds promise as a potential novel therapeutic agent in COPD.

Sirtuins

As discussed previously, SIRT1 is a unique protein deacetylator and, because of its dependence on NAD, it is closely linked to cellular metabolism. Changes in SIRT1 activity by caloric restriction or by agents such as resveratrol or SRT501 hold promise as anti-ageing molecules as they have been shown to increase lifespan via a wide range of processes, including suppressed apoptosis, inflammation and enhanced DNA repair (159).

Resveratrol (3,4′,5-trihydroxystilbene) is a plant polyphenol and appears to mediate its antioxidant and anti-ageing effects partly by activating SIRT1 as well as through inhibition of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), an intracellular lipid kinase involved in diverse pathways that regulate functions such as cell metabolism, survival and polarity (160).

Recently, a new SIRT1-specific activator SRT2172 has been described (161). Nakamaru et al. showed that in smoking mice, SIRT1 activity in lung was decreased but could be reversed by intra-nasal treatment with SRT2172 (162). New studies also indicate that both natural and synthetic sirtuin-activating compounds may work via a shared allosteric mechanism to stimulate sirtuin activity (163). Sirtuins therefore hold promise as a novel anti-ageing molecule, but its effect on COPD patients' needs further exploration.

Phosphodiesterase (PDE)4 inhibitor: Roflumilast

Cyclic adenosine monophosphate (cAMP) phosphodiesterase (PDE) 4 inhibitors, particularly roflumilast, is another potential novel therapy that has been used in COPD patients (164). It has a variety of anti-inflammatory effects including decreasing inflammatory mediators and the expression of cell surface markers as well as inhibition of apoptosis (164). Several clinical trials have shown that PDE4 inhibitors may have beneficial effect over placebos in improving lung function and reducing the frequency of COPD exacerbations (165). The greatest benefit has been observed in patients with moderate-to-severe COPD, particularly the chronic bronchitic phenotype along with a recent history of exacerbations (166). The benefits have been shown in studies, monotherapy as well as in combination, with long-acting beta (2)-agonists and anticholinergic therapy (166). Additive anti-inflammatory effects of corticosteroids and roflumilast have also been postulated, but this needs further exploration (167).

Conclusion

In summary, a myriad of mechanisms including that of oxidative stress, inflammation, cellular senescence, protease injury and apoptosis all play significant roles in accelerated ageing relevant to the pathogenesis of COPD. Basic and translational research has provided important new information that has helped our understanding of these mechanisms to a great extent. This knowledge should lead to development of future new biomarkers to test the relevance of these proposed pathogenic mechanisms in COPD with the opportunity of developing novel therapies targeted to reduce the burden of the inflammatory and oxidative stress load, thus serving as potential anti-ageing therapeutic agents in the cohort. Further studies are therefore needed to understand the effect of these novel therapies in the different phenotypes of COPD and will be of paramount importance to give us more insight to treat this heterogeneous disease more effectively.