COPD is a respiratory illness characterized by progressive and irreversible air flow obstruction. Worldwide, COPD affects an estimated 380 million people [
93] and is the third leading cause of death. Although often referred to as a “disease”, COPD encompasses a spectrum of disorders with two predominant phenotypes: chronic bronchitis and emphysema. Chronic bronchitis predominantly affects the airways and is characterized by mucus hypersecretion, which functionally leads to airway obstruction and a productive cough [
94]. In contrast, emphysema is an anatomical condition characterized by the permanent destruction of the alveolar walls, resulting in parenchymal destruction [
95,
96]. Despite the fact that chronic bronchitis and emphysema can present independently of one another, it is now widely accepted that most cases of COPD typically fall somewhere in the middle of a “COPD-spectrum” and individuals with COPD often exhibit characteristics of both chronic bronchitis and emphysema to varying extents [
94].
Although COPD is predominantly caused by CS, other environmental risk factors include inhalational exposure to ambient (e.g., air pollution) and occupational (e.g., coal mines and pulp and paper manufacturing) toxicants [
97,
98,
99,
100,
101]. Moreover, only 15–20% of smokers go on to develop COPD, indicating that factors beyond exposure to inhalational toxicants are important. This includes genetic factors [
102,
103]. The only established genetic risk factor for COPD is alpha-1 antitrypsin (AAT) deficiency [
104], which occurs in 3–10% of individuals with COPD [
105,
106]. However, COPD is a heterogenous disease with many interrelated pathogenic mechanisms including inflammation, oxidative stress and cell death; there is also a growing body of experimental evidence demonstrating that the AhR attenuates several of these mechanisms that ultimately contribute to the development of this disease.
3.2.1. Inflammation
Cigarette smoking promotes pulmonary inflammation in that the number and proportion of immune cells in the lung shifts in response to CS exposure. Human cigarette smokers have heightened levels of pulmonary neutrophils, macrophages and CD8
+ T-lymphocytes [
107]. These cell types are also increased in mice exposed to CS [
108]. In COPD subjects, the quantity of these cell types are further increased compared to smokers without COPD [
109]. Although macrophages and CD8
+ T-lymphocytes are the predominant inflammatory cell types in the lungs of humans with COPD, neutrophilia is also common [
110]. Elevated neutrophil numbers are also seen in the bronchoalveolar lavage (BAL) of mice exhibiting a COPD-like phenotype [
111].
Neutrophils are the first immune cell type recruited to the lung in response to CS, exhibiting a significant increase in the BAL after only 3-days of CS exposure [
108]. AhR-deficient mice exhibit significantly greater neutrophilia than AhR-expressing mice following both an acute (3 day) [
112] and extended (2–4 week) CS exposure [
11,
12], indicating a role for the AhR in attenuating this early CS-induced neutrophilic response. The mechanism by which this occurs is incompletely understood. One possibility involves the AhR-dependent regulation of the NF-κB protein RelB. The AhR physically interacts with RelB [
113,
114]. Moreover, AhR-deficient mice exhibit a more rapid degradation of pulmonary RelB protein following CS exposure than that observed in CS-exposed AhR-expressing mice [
112]. Increased RelB degradation in AhR-deficient mice is associated with increased expression of the neutrophil adhesion protein ICAM1 and heightened neutrophil infiltration [
112]. These findings suggest the possibility that the AhR may attenuate CS-induced neutrophilia via a non-canonical protein interaction with RelB.
CS-induced macrophage and lymphocyte pulmonary infiltration is delayed relative to neutrophils. Significant increases of macrophage and lymphocyte numbers in the murine BAL occurs after approximately 10-days of CS exposure, whereas significant increases in the lung parenchyma is only observed after several months of CS exposure [
108]. The longest in vivo CS exposure that has been reported using AhR-deficient and AhR-expressing mice is 4-weeks [
11]. Based on this study, it appears that CS-induced macrophage and lymphocyte infiltration in the murine lung is independent of the AhR because although this CS duration induces a significant increase in the number of BAL macrophages and lymphocytes, this increase is independent of AhR expression [
11].
The pro-inflammatory enzyme cyclooxygenase-2 (COX-2) is robustly induced in response to CS exposure [
115] and is elevated in COPD subjects [
10]. COX-2 may contribute to COPD pathogenesis because it initiates the downstream production of several classes of inflammatory mediators such as prostacyclins, prostaglandins (PG) and thromboxanes [
116]. Experimentally, inhibition of COX-2 expression (and the downstream COX-2 product PGE
2) using celecoxib attenuates the development of CS-induced airspace enlargement in the rat lung [
117]. The AhR-mediated regulation of RelB may attenuate other aspects of CS-induced inflammation, such as the production of COX-2. In support of this, RelB reduces the expression of COX-2 [
115]. Another non-canonical AhR-protein interaction that may contribute to the AhR attenuation of CS-induced inflammation is its regulation of Human Antigen R (HuR). HuR is an RNA-binding protein that functions to stabilize target mRNA when localized to the cytoplasm, thereby promoting mRNA translation into protein.
Cox2 is a target mRNA of the HuR [
10]. The AhR attenuates CS-induced COX-2 expression via the nuclear sequestration of HuR, resulting in the destabilization and degradation of
Cox2 mRNA in vitro [
10]. This resulted in reduced
Cox2 mRNA and protein expression in AhR-expressing cells [
10]. The AhR may therefore represent a physiological regulatory mechanism to limit COX-2 overexpression. This is supported by evidence that COX-2 products, such as prostaglandins, act as AhR agonists [
118] and the activated AhR then functions to attenuate CS-induced COX-2 production [
10].
3.2.2. Oxidative Stress
Oxidative stress is another mechanism linked to COPD pathogenesis [
119]. In the healthy lung, reactive oxygen species (ROS)- such as superoxide anions, hydroxyl radicals and hydrogen peroxide- are counterbalanced by the production of endogenous antioxidants, including superoxide dismutase (SOD), catalase (CAT) and the glutathione (GSH)/glutathione peroxidase system [
120]. When ROS production exceeds the capabilities of these antioxidant defenses, oxidative stress ensues. Inhalational exposure to CS results in heightened ROS production in the lungs, as approximately 10
17 oxidant molecules are produced with each puff of a cigarette [
121]. Additionally, ROS production by recruited immune cells (e.g., neutrophils and macrophages) represent another major oxidant source [
120]. Cigarette smokers exhibit oxidative stress-induced lipid peroxidation [
122] and reduced antioxidant capacities (e.g., reduced SOD and CAT activity [
123]). There is evidence to support that AhR attenuates CS-induced oxidative stress. CS-exposed AhR-deficient lung structural cells exhibit significantly higher ROS production compared to CS-exposed AhR-expressing cells [
9]. Additionally, AhR-deficient mouse lung fibroblasts (MLFs) also exhibit an impaired induction of the antioxidants sulfiredoxin 1 (
Srxn1) and NADPH: quinone acceptor oxidoreductase 1 (
Nqo1) following in vitro exposure to CS relative to AhR-expressing MLFs [
9].
However, factors other than strictly CS likely contribute to the oxidative stress observed in COPD. In support of this, greater lipid peroxidation is observed in individuals with COPD that have never smoked relative to subjects without COPD [
124]. Moreover, COPD subjects also have significantly reduced antioxidant expression (e.g., SOD and GSH levels) relative to smokers without COPD [
125]. Reduced AhR expression in COPD-derived lung structural cells [
9] has the potential to increase oxidative stress in the COPD lung. Consistent with this notion, AhR-deficient MLFs exhibit reduced expression of the antioxidants SOD1 and SOD2 relative to AhR-expressing MLFs at baseline [
20]. This illustrates that AhR ablation is associated with a diminished expression of antioxidants that are necessary to combat oxidative stress.
3.2.3. Loss of Lung Structural Cells
A hallmark of the emphysema component of COPD is the loss of lung structural cells [
126,
127,
128]. This includes loss of alveolar epithelial cells responsible for gas exchange and fibroblasts that synthesize the extracellular matrix necessary for lung structure and elasticity. CS induces apoptotic cell death in all major lung structural cell types, including bronchial and alveolar epithelial cells, fibroblasts, endothelial cells and airway smooth muscle cells [
129,
130,
131,
132,
133]. Furthermore, humans with emphysema exhibit heightened pulmonary apoptosis [
134]. Experimentally, intra-tracheal injection of the apoptotic protein cleaved caspase-3 induces epithelial cell apoptosis and airspace enlargement in the murine lung [
126], consistent with the notion that lung parenchymal destruction is linked to cell death.
The AhR has the potential to contribute to loss of structural cells in several ways. CS-exposed AhR-deficient lung structural cells exhibit significantly more apoptosis [
20]. Mechanistically, attenuation of CS-induced apoptosis in vitro is mediated by the AhR-dependent regulation of miR-196a [
135]. Another mechanism may be that loss of lung structural cells in the COPD lung is due to decreased replacement of lost cells. AhR-deficient cells exhibit reduced cellular proliferation relative to AhR-expressing cells [
136]. This is similar to the reduced proliferative capacity of lung fibroblasts from subjects with emphysema [
137,
138]. Cellular senescence may also contribute to loss of lung structural cells in COPD, as it is increased in alveolar epithelial cells from subjects with emphysema and is also positively correlated with more severe airflow obstruction [
139]. Interestingly, cultured AhR-deficient mouse embryonic fibroblasts (MEFs) reach a state of senescence more rapidly than AhR-expressing MEFs [
140]. Collectively, these findings suggest that the AhR may attenuate lung parenchymal destruction by promoting conditions that reduce the loss of lung structural, including altering cell death, proliferative capacities and/or cellular senescence.
3.2.4. Exacerbations
Another important clinical feature of COPD is the occurrence of exacerbations, which are bouts of worsened symptoms that negatively impact quality of life [
141]. COPD exacerbations are largely believed to be from infectious agents [
142], particularly bacterial in origin [
143]. The AhR is implicated in attenuating several bacterial lung infections of relevance to COPD exacerbations. Two of the most commonly-isolated bacteria associated with COPD exacerbations are
Streptococcus pneumoniae (reported in 10–15% of cases) and
P. aeruginosa (reported in 5–10% of cases) [
144]. Experimental evidence has demonstrated that following challenge with
S. pneumoniae, AhR activation by the exogenous ligand TCDD reduces lung bacterial burden and increases survival [
145]. The AhR also helps maintain host defense against
P. aeruginosa. AhR-deficiency results in increased lung bacterial burden and reduced mortality following
P. aeruginosa infection relative to AhR-expressing mice [
41]. Mechanistically, the AhR may contribute to a coordinated immune response against
P. aeruginosa by sensing bacterially produced virulence factors (e.g., pyocyanin), which act as AhR agonists to induce an AhR-mediated production of the neutrophil chemoattractant IL-8 [
146] and the subsequent recruitment of neutrophils [
41].