Chemical constituents of tobacco smoke induce the production of interleukin-8 in human bronchial epithelium, 16HBE cells
© The Author(s). 2016
Received: 18 February 2016
Accepted: 1 July 2016
Published: 19 July 2016
Interleukin-8 (IL-8) functions as a major chemoattractant and plays pivotal roles in the initiation and development of chronic obstructive pulmonary disease (COPD), and tobacco smoke is a most risk factor contributing to the development of COPD. Hence, we have screened some of the tobacco smoke-derived chemical compounds that potentially induce the production of IL-8 in human bronchial epithelium, 16HBE cells.
Twenty-eight hazardous smoke components belonging to 9 classes including nicotine, ammonia, aromatic amines, polycyclic aromatic hydrocarbons, phenols, carbonyls, hydrocyanic acid, nitrosamines and other volatile organics were used in the experiments. Proliferation of 16HBE cells was determined by cell counting kit-8 kit, luciferase activity was measured in IL-8 reporter gene-expressing 16HBE cells, and IL-8 levels in culture supernatants were quantified by enzyme-linked immunosorbent assay.
At the non-toxic dosages, chemical compounds belonging to nicotine, aromatic amines, benzopyrene, phenols, aldehydes, and some other volatile organics dose-dependently increased IL-8 reporter gene expression. Consistently, the representative compounds belonging to nicotine, aromatic amines, benzopyrene, phenols, aldehydes, and some other volatile organics significantly and dose-dependently increased IL-8 levels in the culture supernatants of 16HBE cells, among these compounds, benzopyrene is a most potent stimulator for inducing IL-8 production.
The present study has identified particular tobacco smoke constituents responsible for inducing the IL-8 production in human bronchial epithelium, which might help shed light on the pathogenesis of tobacco smoke-induced COPD.
Chemical compounds used in this study and of Hoffman analytes
Carbon monoxide (CO)*
Hydrocyanic acid (HCN)
Other volatile organics
4-(N-Nitrosomethylamino)-1- (3-pyridyl)-1-butanone (NNK)
As, Se, Hg*
Though it is uncertain whether particular tobacco smoke constituents are responsible for specific adverse health outcomes, there is a broad scientific agreement that several classes of chemical compounds of burned tobacco are toxic and carcinogenic . Tobacco smoke is a most risk factor contributing to the development of chronic obstructive pulmonary disease (COPD), a major cause of morbidity and mortality worldwide . COPD is characterized by a slow and progressive decline in lung function and irreversible airflow obstruction, resulting from chronic airway inflammation, airway remodeling, and emphysema [10, 11]. Bronchial epithelial cells (BEC) not only form a first line of defense against inhaled chemical mixture of combustion compounds, but also initiate and control immune and inflammatory responses in COPD pathogenesis . Upon activation by chemical mixture, BEC are activated and release pro-inflammatory cytokines and chemokines, which in turn recruit inflammatory cells including macrophages, neutrophils and dendritic cells. Activated immune cells further secrete inflammatory mediators, reactive oxygen species and proteolytic enzymes that contribute to the pathogenesis of COPD .
Among pro-inflammatory cytokines, interleukin-8 (IL-8, CXCL8), a member of the CXC chemokine family, plays a pivotal role in COPD pathogenesis [14, 15]. IL-8 is a major chemoattractant to neutrophils, which are responsible for inducing and sustaining the inflammatory state . A close correlation between the number of neutrophils in induced sputum in COPD patients, the IL-8 level, and clinical outcome of the illness has been reported . On the contrary, neutralization of IL-8 by a monoclonal antibody recognizing IL-8 in COPD causes a decrease of exacerbation frequency and clinical symptoms . In the present study, we attempts to determine whether particular tobacco smoke constituents are responsible for inducing the IL-8 production in human normal bronchial epithelium, 16HBE cells.
Forty-four hazardous smoke components of Hoffmann analytes were listed, and 17 of them were not included in the present study due to their instability and unavailability (Table 1). Twenty-seven hazardous smoke components belonging to 9 classes were used in the present experiments. They are nicotine, ammonia, aromatic amines including 1-naphthylamine, 2-naphthylamine, 3-naphthylamine, and 4-naphthylamine, PAH including benzopyrene (BaP), phenols including phenol, catechol, resorcinol, quinol, m-cresol, and p-cresol, carbonyls including methanal, ethanal, propanal, butenal, acetone, and 2-butanone, hydrocyanic acid (HCN), nitrosamines including N’-Nitrosonornicotine (NNN), Nitrosoanatabine (NAT), N-Nitrosoanabasine (NAB), 4-(N-Nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK), and other volatile organics including pyridine, quinoline, and benzene. All of these compounds were of analytic reagent and purchased from Sigma Co. (St. Louis, MO).
Preparation of cigarette smoke extracts
Cigarette smoke extracts (CSE) were prepared according to a standard protocol as described previously . Research-grade cigarettes (3R4F) were purchased from the Kentucky Tobacco Research Council (University of Kentucky, Lexington, KY). The cigarette components labeled on the packaging box of tobacco were as follows: total particulate matter, 10.9 mg/cigarette; tar, 9.4 mg/cigarette; and nicotine, 0.726 mg/cigarette. CSE was prepared by bubbling smoke from three cigarettes into 30 ml phosphate-buffered saline (PBS). CSE was standardized by measuring the absorbance at a wavelength of 320 nm. After filtered through a 0.45-mM filter, CSE were frozen in aliquots and stored at −80 °C. An aliquot of CSE was thawed immediately before use.
Human bronchial epithelial cells, 16HBE cells (generously provided by Professor Huahao Shen, Zhejiang University, Hangzhou, China), were grown in minimal essential medium with Earle’s salts (Life Technologies, Inc., Carlsbad, CA) supplemented with 10 % fetal calf serum (FCS, Life Technologies), 2 mM L-glutamine and 100 units/ml penicillin and streptomycin (Invitrogen, Grand Island, NY). The cells were subcultured before reaching confluence and incubated at 37 °C with 5 % CO2.
Determination of cell proliferation by cell counting kit-8 kit
16HBE cells were seeded in 96-well plates at a density of 2 × 104 cells/well, after pre-culture for 24 h, media were changed and the cells were treated with vehicle or indicated concentrations of chemical compounds for 72 h. After that, the cells were further incubated in the presence of counting kit-8 (CCK-8) solution (10 μM/well, Dojindo Molecular Technologies, Inc. Rockville, MD) for 4 h, and then subjected to absorbance determination by using a multiplate reader at the wavelength of 450 nm. The absorbance form cells treated with vehicle was defined as 100 %, and the absorbance from cells treated with tested compound was normalized to absorbance from cells treated with vehicle.
Construction of IL-8 gene reporter plasmid
A complex human IL-8 promoter region (nt −1475 to nt +1) was amplified by polymerase chain reaction (PCR) in the presence of genomic DNA from 16HBE cells as described previously [20, 21]. The 5’-GCACTCGAGTAACCCAGGCATT ATT-3’ and 5’- GCTAAGCTTAGTGCTCCGGTGGCTTTT-3’ were used as forward and reverse primers, respectively. The PCR product was further cloned into pGL6 reporter plasmid (Promega, Madison, WI) at KpnI and XhoI sites to generate the luciferase reporter construct of IL-8 (pGL6-IL-8-Luc), and the construct was verified by a DNA sequencer.
Generation of 16HBE cells stably expressing IL-8 reporter gene
Stably IL-8 reporter gene-expressing 16HBE cells were generated as described previously [22, 23]. 16HBE cells were seed in a 6-well plate at a density of 1 × 105 cells/well, after pre-culture for 24 h, cells were transfected with pGL6-IL-8-Luc for 24 h in the presence of serum. In each well, 4.8 μl Lipofectamin 3000 reagent, 8 μg pGL6-IL-8-Luc plasmid were used for transfection. After that, cells were selected in the growth medium containing G418 (800 μg/ml; Sigma) for 2 weeks. G418-selected colonies were picked up for expanding, and several clones that were positive for the introduced DNA were monitored by PCR. The clones with high response to 3 % CSE were chosen for the following experiments.
16HBE cells stably expressing IL-8 reporter gene were plated in 24-well plates at a density of 8 × 104 cells/well, after pre-culture for 24 h, cells were treated with vehicle or indicated concentrations of chemical compounds for 48 h. Then, cells were harvested, and the whole cell lysates were prepared in 100 μl reporter lysis buffer (Promega). The supernatants of cell lysates were used for protein quantification by Bradford assays (Beyotime Biotechnology, Shanghai, China) and luciferase assays according to the manufacturer’s instructions (Promega). The luciferase activity was normalized by protein level in the cell supernatants. The luciferase activities derived from vehicle-treated cells were defined as 1.
Determination of IL-8 levels in culture supernatants by ELISA
16HBE cells were plated in 48-well plates at a density of 4 × 104 cells/well, after pre-culture for 24 h, cells were treated with vehicle or indicated concentrations of chemical compounds for 48 h. Then, the cell supernatants were harvested for determination of IL-8 levels by human CXCL8/IL-8 quantikine ELISA Kit (R&D Systems Inc., Minneapolis, MN). The sensitivity of IL-8 assay is 7.5 pg/ml, and the inter- and intra-assay coefficients of variation for IL-8 measurements are 6.7 % and 4.6 %, respectively.
Numerical data were expressed as means ± SD and analyzed by one-way ANOVA and Tukey–Kramer multiple comparisons test. Differences were considered significant when p < 0.05 and p < 0.01. The SPSS statistical package (IBM, North Castle, NY, USA) was used for statistic assays. Experiments were independently repeated at least three times, the inter-experimental coefficients of variation were less than 10.6 %, and the representative results were shown.
Proliferation of 16HBE cells after exposure to chemical compounds
Effects of chemical compounds on transactivation of IL-8 gene
Effects of chemical compounds on the IL-8 levels in culture supernatants
By using in vitro cell culture approaches, we have identified the tobacco smoke-derived chemical compounds capable of inducing the IL-8 production in human bronchial epithelial cells. Among 9 classes, nicotine, PAH, phenols, aromatic amines, aldehydes, and some other volatile organics induce the IL-8 production, and among 27 chemical compounds, BaP is most potent agent to induce the IL-8 production in 16HBE cells.
Thousands of chemical compounds exist in burned tobacco, it is not likely to identify all constituents responsible for stimulation of IL-8 production. We chose 44 hazardous smoke components that were listed in the well-acknowledged Hoffmann analytes [6, 7]. Because inorganic elements exist in combined but not free form in burned tobacco, they were not included in the present study. To screen the chemical compounds potentially inducing the IL-8 expression in human bronchial epithelial cells, we performed IL-8 promoter-driven luciferase assays followed by determination of IL-8 levels in culture supernatants. The cell’s sensitivity to CSE and likely to tested compounds depends upon the confluent state of cells, and cells in higher confluence are less frangible to CSE treatment . Because we have plated cell at a same initial density and treated the cells with non-toxic dosages of chemical compounds for the same period (both of 48 h for reporter and ELISA assays), cells exposed to non-toxic dosages of compounds are supposed to be in the same density and the obtained results from the same type of assay are comparable. Moreover, since the cells respond robustly to the treatment of 3 % CSE, a positive control, in both IL-8 reporter assay and IL-8 protein determination, we suggest that our systems are applicable and robust for assessing the biological effects of chemical compounds from tobacco smoke.
Nicotine, PAH, phenols, aromatic amines, aldehydes, and some other volatile organics induce the IL-8 reporter activities, which well corresponds to their inducing the expression of IL-8 at protein levels, suggesting that increased IL-8 levels could be resulted from up-regulation of IL-8 gene transcription. Induction of IL-8 production by aldehydes well corresponds to previous studies showing that unsaturated rather than saturated aldehydes induce the IL-8 production in human macrophages, small airway epithelial cells, and other human pulmonary cells [25–27]. However, which transcriptional factors are responsible for the transactivation of IL-8 gene in response to these chemical compounds are still unknown. It has been reported that potential binding sites for transcription factors including interferon regulatory factor 1 (IRF-1), Hepatocyte nuclear factor 1 (HNF-1), activator protein 1 (AP-1), CCAAT/Enhancer Binding Protein (C/EBP), Nuclear factor-kB (NF-kB) present in the region of −133 kb to −85 kb of IL-8 promoter, which is necessary for activation by some chemical compounds [25, 26]. Moreover, podechard et al. have found that BaP up-regulates IL-8 at mRNA and protein levels through aryl hydrocarbon receptor (AhR)-dependent transactivation of IL-8 gene in primary human macrophages , corresponding to our findings showing that BaP is a most potent stimulator among the chemical compounds we have investigated. Thus, we suggest that the chemical compounds of tobacco smoke transactivate the IL-8 gene and up-regulate the IL-8 expression possibly through activation of these transcriptional factors. Further experiments are required to address this issue.
Interestingly, all aromatic amines including 1-naphthylamine, 2-naphthylamine, 3-naphthylamine, and 4-naphthylamine led to significant induction of IL-8 reporter expression, all phenols including phenol, catechol, resorcinol, quinol, m-cresol, and p-cresol have significant effect on IL-8 reporter expression, and all aldehydes including methanal, ethanal, propanal, and butenal also robustly induce IL-8 reporter expression. On the contrary, all nitrosamines including NNN, NAT, NAB, and NNK have no obvious effects on IL-8 reporter expression. These findings support the notion that genre compounds possess the same property in induction of IL-8 reporter expression and IL-8 production. However, possibly due to no significant structural difference among the genre compounds, structure-activity relationship has not been found in our experiments. Though most of chemical compounds existing in tobacco smoke are at milligram, microgram, or nanogram levels that are significantly lower than dosages we used in the present study [6, 28], we suggest that the present study is still of great value according to the following reasons: (1) long-term exposure of chemical compounds to smokers results in their accumulation in bronchial epithelium and subsequent augmentation of their toxicity; (2) potential synergism among these compounds amplifies their toxicity; (3) unlike exposure of chemical compounds to the in vitro cultured cells in the form of solution, direct and successive inhalation of chemical compounds into bronchial epithelium of smokers possibly aggregates their toxicity. Thus, further experiments on the in vitro and in vivo effects of these chemical compounds are worthy of investigation.
Given the fact that tobacco smoke is a most risk factor contributing to the development of COPD, a major cause of morbidity and mortality worldwide , to identify the clinical significance of these hazardous compounds in the initiation and development of COPD is of great importance. Given the fact that decreases in inhalation of hazardous compounds benefit to not only smokers but also the people in polluted environments, our findings may have significant implications in specific adverse health outcomes.
Taken together, the present study has identified several of the major classes of chemical mixture of burned tobacco responsible for inducing the IL-8 production, among 27 chemical compounds, BaP is one of most potent IL-8 inducer in human bronchial epithelium. Thus, the present study might help shed light on the pathogenesis of tobacco smoke-induced COPD.
This program is supported by Science and Technology Department of Zhejiang Province (2013C03050 to G. Zhou).
GZ and WX was involved in the analysis, literature review and drafted introduction, results, and discussion piece of the article. XmW provided insights on conceptualization of the article. He provided inputs in framing rationale, discussion and conclusions section of the article. CX, YH, XkW, FH, XL and CS participated in its design and data collection and helped to draft the manuscript. All authors contributed in writing and critical review of the manuscript. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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