Airway Basal Stem/Progenitor Cells Have Diminished Capacity to Regenerate Airway Epithelium in Chronic Obstructive Pulmonary Disease

Abstract

Cigarette smoking, and the eventual development of chronic obstructive pulmonary disease (COPD) in susceptible individuals, is characterized by disordered differentiation of the small airway epithelium (SAE) (1, 2). The SAE is composed of four major cell types, the differentiated ciliated and secretory cells responsible for lung defense and the undifferentiated intermediate and basal cells that function as the stem/progenitor cells of the ciliated and secretory cells (3,4). On the basis of the knowledge that hyperplasia of the basal cell population is the first biologic abnormality associated with smoking (5) and that the basal cells are the stem/progenitor cells of the SAE (4), we hypothesized that smoking and, to a greater extent, COPD are associated with basal cell "fatigue," resulting in reduced capacity of the basal stem/progenitor cells to successfully differentiate into a normal mucociliary epithelium. In addition, as DNA methylation is known to mediate renewal and differentiation capacities of adult tissue stem cells (6) and cigarette smoking is associated with changes in DNA methylation in lung cells (7,8), we further hypothesized that basal cells from the SAE that fail to differentiate to a mucociliary epithelium have altered DNA methylation patterns compared with SAE basal cells capable of differentiating in a normal fashion.To assess these hypotheses, we cultured primary basal cells from human SAE sampled by bronchoscopic brushing of 47 individuals (17 healthy nonsmokers, 14 healthy smokers, 16 COPD smokers) and assessed the capacity of the purified basal cells to differentiate in air-liquid interface (ALI) culture. The purified basal cells from each clinical phenotype were plated on type 4 collagen in a chamber well system, as previously described (9). After 2 days, culture media was removed from the apical chamber, exposing the cells to air and inducing differentiation over the course of 28 days (9, 10).Because tight junction formation is associated with successful epithelial differentiation (11), transepithelial electrical resistance, a measure of tight junctional barrier integrity, was determined at Day 10 of ALI culture ( Figure 1A). ALI cultures derived from SAE basal cells from smokers with COPD had significantly lower transepithelial electrical resistance compared with samples derived from healthy nonsmokers (smokers vs. nonsmokers, P . 0.05; COPD smokers vs. nonsmokers, P , 0.003; smokers vs. COPD smokers P . 0.3), which is consistent with previous observations that tight junction integrity is decreased and permeability is increased in the airway epithelium of smokers with and without COPD (12, 13). Furthermore, SAE basal cells from COPD smokers and, to a lesser extent, SAE basal cells from "healthy" smokers were limited in their ability to regenerate a fully differentiated epithelium under standard ALI model conditions, as assessed by pairwise comparison of survival curves using a log-rank test ( Figure 1B; smokers vs. nonsmokers, P . 0.09; COPD smokers vs. nonsmokers, P , 0.009; smokers vs...

Human lung epithelial cell cultures for analysis of inhaled toxicants: Lessons learned and future directions

“…The advantage of this option is underlined by the observation that ALI-cultured airway epithelial cells derived from patients with mild or severe asthma retain characteristics of the distorted airway epithelium present in the patients lungs, and that the phenotypic differences relate to the severity of asthma (Gras et al, 2012). Similar observations have been reported in COPD (Amatngalim et al, 2017;Staudt et al, 2014), suggesting that the lung microenvironment in these inflammatory lung diseases causes (epigenetic) changes in epithelial cells resulting in partial persistence of the phenotype in culture.…”

Lung ageing and COPD: is there a role for ageing in abnormal tissue repair?

“…Stem cell niches fail to respond effectively to additional demands for cell turnover; moreover, deranged metabolic signalling and premature senescence might occur [141]. In line with this, reduced regenerative capacity of basal progenitor cells has been reported in COPD [135]. In addition, in a different study, an abnormal population of TRP63 + KRT5 + KRT14 + basal cells was identified in regions of hyperplasia from sections of COPD human airways [142], suggesting abnormal stem cell function.…”

“…Airway basal cells represent a well-characterised stem cell population located in the trachea and bronchi. These cells have the ability to self-renew and give rise to secretory, ciliated and neuroendocrine cells [135]. In the distal lung, alveolar type II cells [136] have been shown to be able to replenish lost alveolar type II cells and transdifferentiate to alveolar type I cells, thus ensuring proper gas exchange [132,133].…”

Airway Basal Cells. The “Smoking Gun” of Chronic Obstructive Pulmonary Disease

“…Staudt and colleagues (62) demonstrated that when basal cells obtained from the small airway epithelium are placed on air-liquid interface and allowed to differentiate to a mucociliated epithelium over 28 days, 88% of samples isolated from healthy nonsmokers successfully regenerated airway epithelium, but only 64% of those from normal smokers and only 44% of those from smokers with COPD regenerated. Inability to form mechanically stable and properly differentiated airway epithelium by basal cells of smokers with COPD may underlie decreased host defense function and barrier integrity of the airway epithelium in patients with COPD, potentially leading to infection and inflammation, which contribute to progression of COPD as chronic disease.…”