Background
Chronic obstructive pulmonary disease (COPD) is a progressive degenerative lung disease and currently the fourth leading cause of death worldwide [
1]. Cigarette smoke and other toxic particles are major risk factors for the development of COPD. A critical feature of COPD is airway remodeling, which is characterized by aberrant repair of the epithelium and accumulation of fibroblasts [
2,
3]. As airway epithelium is the primary target of inhaled harmful particles, abnormal tissue repair has become a primary focus in understanding the process of airway remodeling [
4].
The airway epithelium consists of four types of cells, including basal cells (BCs), ciliated cells, secretory cells and neuroendocrine cells [
5]. BCs include multipotent stem/progenitor progenitor cells of bronchial airway epithelium and make a major contribution to the regeneration of bronchial epithelium [
6,
7]. These cells play a key role in the maintenance of the normal airway epithelial architecture through their capacity to self-renew, differentiate into ciliated and secretory cells, and establish interactions with mesenchymal cells [
8‐
11]. Abnormalities in the number and function of BCs may therefore severely affect the regeneration of injured airway epithelium such as in smoking. Basal cell hyperplasia caused by smoking has occurred long before lung function declines, indicating that the development of COPD may begin with these cells [
12]. However, little is known about the molecular mechanisms underlying the abnormal biological behavior of BCs in COPD.
Trophoblast cell surface antigen 2 (TROP2), is a type I transmembrane glycoprotein with low to no expression in normal tissues. It provides crucial signals for cells with requirements for self-renewal, survival, and invasion [
13,
14]. Although TROP2 has been reported to be highly expressed in various types of epithelial cancers, including colorectal cancer, pancreatic cancer, and oral squamous-cell carcinoma [
15‐
18], TROP2 expression has also been found in stem cells in various tissue types. In human and mouse prostate, the TROP2 expressing subpopulation of BCs possess stem cell capacities such as self-renewal, regeneration and differentiation [
19,
20]. Undifferentiated oval cells express TROP2 shortly after activation due to liver injury [
21]. TROP2 is also enriched in endometrial-regenerating cells in a dissociated cell tissue recombination assay [
22].
These results indicate that TROP2 might play an important role more generally in the regulation of the growth and regeneration of stem cells as well as serve as a molecular marker of them in various adult tissues. Here, protein expression of TROP2 was specifically examined in the development of COPD in smokers. TROP2 was furthermore overexpressed in normal airway BCs in vitro in order to identify potential biological and molecular pathways regulated by the protein. Our results demonstrate that increased expression of TROP2 occurs in the BC compartment of lung tissue samples, indicating that the protein might play a role in aberrant airway repair and remodeling that are characteristic of COPD.
Methods
Ethics statement
Protocols for recruitment of patients and healthy, nonsmoking individuals as well as the collection of tissue samples and the analysis of patient data were approved by the ethics committee at Qilu Hospital of Shandong University (Jinan, Shandong, China). Written informed consent was obtained from each individual for participation in the study.
Patients
Lung tissue samples were obtained from patients (
n = 72) at Qilu Hospital of Shandong University (Jinan, Shandong, China) during lobectomy or pneumonectomy performed for medical reasons, including lung tumor, pneumatocele and pulmonary cyst. No patient had received corticosteroids (oral or inhaled) within 1 month before tissue collection. The cohort consisted of smokers with COPD (
n = 24), smokers without COPD (
n = 24), and nonsmokers (
n = 24) as controls. The diagnosis of COPD was made according to the guidelines of the Global Initiative for Chronic Obstructive Lung Disease (GOLD) [
1]. Tissue samples of ~15 - 25 mm in size (
n = 2) were removed from the subpleural parenchyma of the lobe at a distance of ≥ 5 cm from the margins of the diseased areas in each patient. The inclusion criterium for smokers with and without COPD was a smoking index of > 10 pack-years which was calculated by multiplying the number of cigarette packs consumed per day (20 cigarettes per pack) with the number of years of smoking. Exclusion criteria were atopic diseases, allergic rhinitis, asthma, diabetes, acute or chronic infections.
Immunohistochemistry and immunofluorescence
Serial sections (4 μm) were cut from paraffin embedded lung samples, and antigen retrieval was carried out for both immunohistochemistry and immunofluorescence by steaming sections for 15 min in Citrate buffer (pH 6.0). Immunostaining and visualization was performed with the Biotin-Streptavidin HRP Detection Systems (SP) and a DAB color development kit (ZhongShan Golden Bridge Biotechnology, Beijing, China), and the primary antibodies used were TROP2 (5 μg/ml; RD Biosciences, Temecula, NJ, USA) and Ki67 (1:100; Abcam, Cambridge, MA, UK). The mean staining density of TROP2 in airway epithelium was analyzed using Image-Pro Plus 6.0 software. For immunofluorescence, sections were incubated overnight at 4 °C with mixed primary antibodies, including anti-TROP2 (10 μg/ml) and anti-cytokeratin 5 (1:200; Abcam) as a marker for basal cells. For visualization, fluorescently-labeled secondary antibodies were incubated with slides, and images were captured with a Laser confocal microscope (Olympus DP70 CCD camera, Tokyo, Japan). TROP2 was expressed as mean staining density in the airway epithelium. Ki67 index was calculated as the percentage of Ki67 positively stained cell compared with the airway epithelial cells. The area of the airway epithelium and the number of epithelial cells were evaluated using Image-Pro Plus 6.0 software (Media Cybernetics, Silver Spring, MD, USA). The analysis of all slides was performed by an experienced lung pathologist.
Isolation and culture of BCs
The primary epithelial cells were derived from healthy donors (
n = 6) and COPD patients (
n = 4) (50 % ≤ FEV1 < 80 % predicted). All the donors were off corticosteroid use in recent month. Airway epithelial cells were obtained by gently brushing the airway epithelium from the third- to fourth-order bronchi of healthy nonsmoking volunteers using flexible bronchoscopy as previously described [
23]. Cells were collected and cultured in a T25 cell culture flask with BEGM (Lonza, Basel, Switzerland) in 5 % CO2 in a humidified chamber at 37 °C. The medium was changed after the first 12 h in culture to remove unattached cells and thereafter, every 2–3 days. Experiments were performed on BCs at passage 2 or 3.
Cyto-immunofluorescence for the identification of BCs
After the first passage, BCs were seeded onto glass cover slips in a 24-well plate and incubated for 24 h. Cells were fixed in 4 % paraformaldehyde for 15 min and blocked with normal goat serum for 30 min at 37 °C. Cells were rinsed thoroughly with phosphate buffered saline (PBS) and incubated with primary antibody against Cytokeratin 5 (1:100; Abcam) and p63 (1:100; ZhongShan Golden Bridge Biotechnology) overnight at 4 °C. Slides were rinsed, incubated with TRITC conjugated anti-rabbit IgG (1:500; Beyotime, Beijing, China) and FITC conjugated anti-mouse IgG at room temperature for 30 min, counterstained with DAPI, and visualized with confocal fluorescence microscopy.
Transfection
BCs were plated at a density of 105 cells/well on a 24-well plate. After 24 h, cells were transfected with pcDNA3.1-TROP2 or siRNA sequence (5′-GCACGCUCAUCUAUUACCUTT-3′, 5′-AGGUAAUAGAUGAGCGUGCTT-3′) using Lipofectamine™ 2000 Transfection Reagent (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. Cells transfected with empty vector or scrambled siRNA were considered as negative control. At 48 h, cells were trypsinized, and experiments were performed.
Quantitative real-time PCR
Total RNA was extracted from harvested cells using TRIzol reagent (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. First strand cDNA was synthesized from RNA (1 μg) using the ReverTra Ace® qPCR RT Kit (TOYOBO, Osaka City, Japan). Real-time PCR reactions (20 μL) were prepared using 2 μL of cDNA in SYBR® Green Real time PCR Master Mix (TOYOBO), and amplification was performed on an ABI PRISM 7900 HT Sequence Detection System (Applied Biosystems, Foster City, CA, USA). Relative quantification of mRNA was performed using the comparative CT method where endogenous GAPDH was used as the normalization control. The primer sequences were as follows: TROP2 forward primer, 5′-CGGCAGAACACGTCTCAGAAG-3′; reverse primer, 5′-CCTTGATGTCCCTCTCGAAGTAG-3′; GAPDH forward primer, 5′-GCACCGTCAAGGCTGAGAAC-3′; reverse primer, 5′-TGGTGAAGACGCCAGTGGA-3′. Results were obtained from three independent experiments performed in triplicate.
Western blot analysis
Cell extracts were prepared by lysing cells in ice-cold RIPA buffer (20 mM sodium phosphate, 150 mM NaCl, pH 7.4, 1 NP-40, 0.1 SDS and 0.5 % deoxycholic acid) containing the protease inhibitor PMSF (Beyotime). Proteins (40 μg) were separated on 10 % SDS PAGE and transferred to PVDF membranes, and the blots were incubated with antibodies against human TROP2 (1 μg/ml), cyclin D1 (1:1000; Beyotime), E-cadherin (1:50000; Abcam), vimentin (1:500; Abcam) and GAPDH (1:2000; GoodhereBiotech Co., Hangzhou, China) as the protein loading control. Experiments were carried out in triplicate and repeated between three to five times.
Cell proliferation and viability assay
Cell viability was assessed using a colorimetric assay, Cell Counting Kit-8 (CCK-8), according to the manufacturer’s instructions (Bestbio, Shanghai, China). Briefly, transfected and control cells were seeded onto 96-well plates at a density of 8000 cells/well and incubated in 5 % CO2 in a humidified chamber at 37 °C for 24, 48 or 72 h. At designated time intervals, CCK-8 solution (10 μL) was added to each well, and the optical density (O.D.) was measured at 450 nm in a microplate reader (Bio-Rad Model 680, Richmond, CA, USA) after a 3 h incubation at 37 °C. Experiments were repeated three times and six parallel holes were set in each experiment.
Cell cycle analysis
Cells were transfected with pcDNA3.1-TROP2 or with siRNA sequence, harvested 48 h after transfection by trypsinization, and fixed in 75 % cold ethanol for 1 h at −20 °C. The cells were pelleted, rinsed with PBS, and incubated with 100 μL RNase A (100 mg/mL) and 400 μL propidium iodide for 30 min at 37 °C. Cell cycle analysis was performed on the FACSCalibur flow cytometer (Becton Dickinson, San Jose, CA, USA) at 488 nm. The relative ratios of the G1, S, and G2 phases were analyzed with ModFit LT 4.0 software. The assay was done in triplicate and repeated in three independent experiments.
Wound repair
BCs were seeded onto 6-well plates, incubated overnight, and transfected with pcDNA3.1-TROP2 or with siRNA sequence. After reaching 90 % confluency, cell monolayers were scratched with a sterile pipette tip. Floating cells were removed with PBS and reseeded in BEGM medium. Images were taken at 0, 24 and 48 h to document the rate of migration of cells into the wound. Results were expressed as the ratio of the wound area detected at the designated time interval relative to the original scratch area. Repair was quantified using Image-Pro Plus software (Media Cybernetics, Rockville, MD, USA). Experiments were carried out in triplicate and repeated three times.
Enzyme linked immunosorbent assay (ELISA)
Cells were transfected with pcDNA3.1-TROP2 or with siRNA sequence. The supernatants were harvested 48 h later, and the levels of IL-6, IL-8, and IL-1β were quantified in the supernatants using an ELISA kit according to the manufacturer’s instructions (RD Biosciences). The ELISA assay results were obtained from three independent experiments performed in triplicate.
Statistical analysis
SPSS version 18.0 (SPSS Inc.; Chicago, IL, USA) was used to perform statistical analysis. All data were expressed as the mean ± the standard deviation (SD). The Kruskal–Wallis and Mann–Whitney U-tests were used for comparisons between patient groups, and correlation analyses were performed with Spearman rank correlation. The Student’s t test was used for analysis of in vitro experiments. P < 0.05 was considered statistically significant.
Discussion
Airway BCs play crucial roles in epithelial regeneration and homeostasis following injury [
24]. Disregulated BC function has been demonstrated to be an early event in the pathogenesis of COPD, based in part on changes in BC transcription in smokers and as the smoker transitions into clinically defined COPD [
25‐
27]. BC hyperplasia has been a histologic abnormality associated with smoking [
28], although the reasons remain obscure. Because of its association with proliferation and progenitor cells in mouse and human, TROP2 was the focus of our study as a potential regulator of the biological behavior of airway BCs in COPD.
Our results demonstrated that TROP2 expression was significantly increased in vivo in airway BCs of COPD patients, and revealed a significant correlation between TROP2 expression and the degree of airway epithelium hyperplasia. In vitro, TROP2 promoted BC proliferation and self-renewal mediated possibly through the ERK signaling pathway. Finally, TROP2 induced an EMT-like phenotype and release of proinflammatory cytokines in proliferating BCs. These results indicate that increased TROP2 protein in BCs might play a critical role in airway remodeling which is a key pathological feature of COPD in patients.
Regulation of TROP2 gene expression has been detected by DNA microarray analysis in cancer cells [
29]. Whereas little is known about the mechanisms that could be responsible for the increased expression of TROP2 in COPD epithelium. In our study, staining intensity was generally greater in smokers regardless of COPD status than in nonsmokers, indicating that smoking alone was a risk factor up-regulating TROP2 expression. Smoking was not however the single contributing factor, as higher TROP2 expression was specifically observed in patients with COPD. One explanation for this result might be an individual genetic susceptibility to COPD. Further research is necessary to investigate the exact regulatory mechanism of TROP2 expression in airway BCs. Further, the relationship between TROP2 and cell proliferation due to the expression of the proliferation-related nuclear protein Ki67 was established in this study. The significant correlation between TROP2 and Ki67 expression supports the hypothesis that overexpressed TROP2 is closely related to the hyperproliferative state of airway epithelium in COPD.
The results from immunohistochemistry were further supported by experiments performed in an in vitro system, which revealed an enssential role that TROP2 played in airway BC proliferation. It has been reported that U0126 (20 μM) is a more highly selective inhibitor of MEK1/2, although it might have a very weak inhibitory effect on PKC, Cdk2 and Cdk4 [
30]. Therefore, our result suggested that pERK1/2 MAPK signal pathway might be involved in the process of TROP2-mediate BC proliferation. The mechanism of TROP2-induced activation of pERK1/2 in BCs is unclear. It has been reported that overexpressed TROP2 could increase the intracellular Ca
2+ level, which was capable of activating the ERK MAPK pathways through calmodulin-dependent protein kinases [
13]. TROP2 regulated cell self-renewal and proliferation through a mechanism of regulated intramembrane proteolysis was found in prostate stem cells [
14].
BC hyperplasia is a typical airway epithelial remodeling phenotype in COPD lungs. The proliferation of BCs can act as a benign protective cellular reaction to injury in the airways of normal individuals, but it may also be the cause of pathological changes occurring at an early stage in COPD. BC hyperplasia in small airways could lead to airway wall thickening, contributing to small airway narrowing and airflow limitation [
31]. Furthermore, the increased number of BCs also gives rise to squamous metaplasia in central airways, which is also an early airway pathological change observed in COPD patients [
32,
33].
BCs acquired EMT-like features during TROP2 mediated proliferation, which was manifested by the decreased expression of epithelial cell markers cytokeratin 5 and E-cadherin and the acquisition of the mesenchymal cell marker vimentin. Classical EMT refers to the epithelial cells closely connected to the basement membrane which lose polarity and acquire a mesenchymal cell phenotype [
34]. The EMT process is present in bronchial epithelial cells of patients with COPD and has a role in airway remodeling [
35,
36]. Our findings indicate that BCs may be one of the cell types undergoing EMT. Consistent with this result, is that BCs have been reported to undergo EMT-like changes in an acute lung injury model of BC differentiation which was mediated by EGF. Airway epithelium generated from the EGF-exposed BCs also exhibited decreased barrier integrity [
37]. In addition, Jonsdottir
et al. recently reported that some p63-positive BCs are prone to phenotypic changes and could act as EMT progenitors in idiopathic pulmonary fibrosis [
38].
Exposure to repetitive inflammatory stimuli leads to exaggerated and persistent activation of the repair process and is one of the primary causes of airway remodeling characteristic of COPD [
39,
40]. In our study, secretion of proinflammatory cytokines IL-6, IL-8, and IL-1β was increased in TROP2 mediated proliferation of BCs. Elevated levels of these proinflammatory factors have been confirmed in the airways of COPD patients. These cytokines serve as effective chemokines, leading to the infiltration of neutrophils and macrophages and the expansion of persistent inflammatory injury to airway epithelium [
41]. BCs are not just cellular targets of the process of inflammation; BCs also release chemokines and cytokines, thereby initiating and orchestrating immune and inflammatory responses. These data support the concept that the pathological change secondary to altered BC proliferation leads to abnormal innate immunity, potentially contributing to the pathologic cycle of chronic airway inflammation and remodeling in COPD.
One limitation of this study is that all the lung tissue samples were collected from patients with mild to moderate COPD, and severe COPD patients incapable of undergoing surgery operation were not included. The situation in severe COPD may be very different from the relative early stage, in which the repairing ability of BCs may be relative conserved. As Perotin et al. reported that delayed rather than enhanced epithelial wound repair was found in severe COPD [
42]. Actually, we agree with this idea. The explanation for this observation may be that the renewal and regeneration capacity of BCs could diminish due to excessive stem cell hyperproliferation and eventual replicative senescence [
43,
44]. And we also observed a so-called aging of pcDNA3.1-TROP2-BCs in culture in cells passaged > 5 generations. This finding parallels the in vivo situation of COPD: an initial promotion of BC proliferation followed by senescence or death after extended time in the repair state. In addition, the mechanism involved in TROP2 inducing EMT-like changes and enhanced proinflammatory cytokines secretion has not been deeply researched yet. pERK1/2 MAPK signal pathway was considered in the study. After pre-treated with U0126, neither EMT makers nor proinflammatory cytokine concentrations underwent significant changes in pcDNA3.1-TROP2-BCs (data not shown), suggesting that pERK1/2 MAPK pathway may not participate in the process. It is necessary to carry on further researches in the subsequent studies.
Acknowledgements
The authors thank the volunteers of the study for their willingness to contribute to our medical research.