Enhanced pulmonary epithelial replication and axial airway mucosubstance changes in F344 rats exposed short-term to mainstream cigarette smoke

Mainstream Cigarette Smoke Impacts the Mouse Vocal Fold Epithelium and Mucus Barrier

OBJECTIVES/HYPOTHESIS

Cigarette smoke (CS) is a primary risk factor for the development of numerous benign and malignant laryngeal diseases. The epithelium and mucus lining the vocal folds (VF) are the first barriers against CS. The primary objective of this study was to investigate epithelial and mucus barrier changes in the mouse laryngeal mucosa upon exposure to subacute CS. The secondary objective was to compare mucus barrier changes in mice and human smokers and nonsmokers. Study Design Animal model.

METHODS

Mice were exposed to CS for 4 weeks for 4 hours (N = 12, high dose [HD]) or 1 hour (N = 12, low dose [LD]) per day. Air-exposed mice were used as a control group (N = 10). Larynges were harvested and VF epithelial barrier integrity was evaluated including cellular proliferation and expression of cell junctions. We also investigated mucus production by examining mucus cell area and mucin expression in mice and human smokers and nonsmokers.

RESULTS

HD CS increased VF epithelial cellular proliferation but did not alter the expression of cell junctions. HD CS also induced hypertrophy of the mucus-producing submucosal glands. However, only LD CS increased MUC5AC gene expression. MUC5AC staining appeared elevated in laryngeal specimens from smokers, but this was not significant as compared to nonsmokers.

CONCLUSIONS

These findings help us identify potential adaptive mechanisms to CS exposure as well as set the foundation for further study of key aspects of epithelial and mucus barrier integrity that may be implicated in laryngeal disease development following prolonged smoking.

LEVEL OF EVIDENCE

NA Laryngoscope, 131:2530-2539, 2021.

Short-term whole body cigarette smoke exposure induces regional differences in cellular response in the mouse larynx

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Short-term CSE induced regional differences in murine laryngeal cellular responses.

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Basal cell hyperplasia accompanies adaptive cell proliferation in the vocal folds.

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Increased subglottic cell proliferation persists even after CS cessation.

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SEM revealed vocal fold microprojection damage with possible necrotic features.

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Subglandular acidic mucins decreased and neutral mucins increased post-CSE.

The larynx is an essential organ in the respiratory tract and necessary for airway protection, respiration, and phonation. Cigarette smoking is a significant risk factor associated with benign and malignant laryngeal diseases. Despite this association, the underlying mechanisms by which cigarette smoke (CS) drives disease development are not well elucidated. In the current study, we developed a short-term murine whole body inhalation model to evaluate the first CS-induced cellular responses in the glottic [i.e. vocal fold (VF)] and subglottic regions of the larynx. Specifically, we investigated epithelial cell proliferation, cell death, surface topography, and mucus production, at various time points (1 day, 5 days, 10 days) after ∼ 2 h exposure to 3R4F cigarettes (Delivered dose: 5.6968 mg/kg per cigarette) and following cessation for 5 days after a 5 day CS exposure (CSE). CSE elevated levels of BrdU labeled proliferative cells and p63 labeled epithelial basal cells on day 1 in the VF. CSE increased proliferative cells in the subglottis at days 5, 10 and following cessation in the subglottis. Cleaved caspase-3 apoptotic activity was absent in VF at all time points and increased at day 1 in the subglottis. Evaluation of the VF surface by scanning electron microscopy (SEM) revealed significant epithelial microprojection damage at day 10 and early signs of necrosis at days 5 and 10 post-CSE. SEM visualizations additionally indicated the presence of deformed cilia at days 5 and 10 after CSE and post-cessation in the respiratory epithelium lined subglottis. In terms of mucin content, the impact of short-term CSE was observed only at day 10, with decreasing acidic mucin levels and increasing neutral mucin levels. Overall, these findings reveal regional differences in murine laryngeal cellular responses following short-term CSE and provide insight into potential mechanisms underlying CS-induced laryngeal disease development.

Effect of cigarette smoke exposure and mutant Kras overexpression on pancreatic cell proliferation

Pancreatic cancer is the fourth leading cause of cancer-associated mortality. The major risk factor for pancreatic cancer is cigarette smoking. Kras mutations are commonly observed in human pancreatic cancers. The present study examined the hypothesis that exposure to cigarette smoke and overexpression of a mutant Kras gene in the pancreas affects pancreatic cell proliferation in mice. Mice overexpressing the mutant Kras gene (KRas^G12D) in the pancreas as well as wild-type mice were exposed to environmental tobacco smoke for 2 weeks. Overexpression of mutant Kras increased cell proliferation in pancreatic ductal, acinar and islet cells. Notably, cigarette smoke exposure decreased cell proliferation in pancreatic ductal and acinar cells, and had no effect in islet cells. Cigarette smoke did not affect pancreatic protein levels of tumor necrosis factor (TNF)α, p53, or cyclin D₁, but mutant Kras overexpression slightly decreased TNFα and p53 protein levels. Therefore, pancreatic cell proliferation in mice overexpressing mutant Kras is associated with the later development of pancreatic tumors, but effects of cigarette smoke on pancreatic cell proliferation do not provide a good model for human pancreatic carcinogenesis.

Correlation of Cigarette Smoke-Induced Pulmonary Inflammation and Emphysema in C3H and C57Bl/6 Mice

Cigarette smoke (CS)-exposed mice have been used to model airway inflammation and emphesema in humans; however, the impact of exposure duration, sex, and strain differences in susceptibility to progression of airway inflammation and to emphesema are poorly investigated. This study was designed to determine the association between inflammation and emphysema by exposing 2 strains of mice, C3H/HeN (C3H) and C57BL/6 (Bl/6), to filtered air (FA) or CS for 10, 16, or 22 weeks. Both genders and strains of CS-exposed mice developed pulmonary inflammation as characterized by cell counts in the bronchoalveolar lavage fluid (BALF) and the levels of matrix metalloproteinases (MMPs) in the BALF. CS exposure caused persistently higher number of BALF macrophages in C3H compared to BL/6 mice, while more BALF neutrophils and persistently higher MMP-2 and MMP-9 levels were observed in BL/6 mice. The mean linear intercept (Lm) increased progressively by 26%, 33%, and 55% at 10, 16, and 22 weeks, respectively, in CS-exposed C3H mice compared to the matched air controls. In BL/6 mice, although CS exposure also increased the Lm compared to FA controls, no further increase in Lm beyond the levels observed at 16 weeks of exposure was observed by 22 weeks. These findings suggest that extent of inflammation is not associated with severity of emphysema and underscores the importance of carefully selecting the mouse strains and endpoints when exploring effective treatments for emphesema.

Effect of dietary selenium and cigarette smoke on pulmonary cell proliferation in mice

The objective of this study was to determine if dietary selenium could inhibit pulmonary cell proliferation in control and cigarette smoke-exposed female A/J mice. Selenium in the form of sodium selenite was supplemented to purified diets similar to the AIN-93M diet to yield 0.15, 0.5, or 2.0 mg selenium/kg diet. After 3 weeks, mice in each dietary group were divided into two subgroups; one used as control, whereas the other was exposed to cigarette smoke for five consecutive days. Mice from both groups were euthanized 3 days later. Mice were administered bromodeoxyuridine in the drinking water starting 5 days before the initiation of the smoke exposure and continuing until they were euthanized. After euthanasia, the left lung lobe was processed for histology and cell proliferation analysis. Cigarette smoke increased cell proliferation in the terminal bronchioles and large airways, but not in alveoli. High-selenium diets inhibited cell proliferation in the alveoli, terminal bronchioles and large airways areas in both control and smoke-exposed mice. Increasing the dietary selenium level led to increased selenium levels in the blood and lung, and increased glutathione peroxidase (GPx) activity in the lung. Cytochrome P-450 1A1 protein levels in the lung were increased by cigarette smoke but were not affected by dietary selenium. It is concluded that dietary selenium inhibits pulmonary cell proliferation in both control and cigarette smoke-exposed mice, indicating that selenium is inhibiting cell proliferation independently of smoke exposure, and that this inhibition may be related to selenium concentration and GPx activity in the lung.

Modulators of cigarette smoke-induced pulmonary emphysema in A/J mice

Mice develop pulmonary emphysema after chronic exposure to cigarette smoke (CS). In this study, the influence of gender, exposure duration, and concentration of CS on emphysema, pulmonary function, inflammation, markers of toxicity, and matrix metalloproteinase (MMP) activity was examined in A/J mice. Mice were exposed to CS at either 100 or 250 mg total particulate material/m(3) (CS-100 or CS-250, respectively) for 10, 16, or 22 weeks. Evidence of emphysema was first seen in female mice after 10 weeks of exposure to CS-250, while male mice did not develop emphysema until 16 weeks. Female mice exposed to CS-100 did not have emphysema until 16 weeks, suggesting that disease development depends on the concentration and duration of exposure. Airflow obstruction and increased pulmonary compliance were observed in mice exposed to CS-250 for 22 weeks. Decreased elasticity was likely the major contributor to airflow obstruction because substantial remodeling of the conducting airways, beyond mild mucous cell hyperplasia, was lacking. Exposure to CS increased the number of macrophages, neutrophils, lymphocytes (B cells and activated CD4- and CD8-positive T cells), and activity of MMP-2 and -9 in the bronchoalveolar lavage fluid (BALF). Treatment with antioxidants N-acetylcysteine or epigallocatechin gallate (EGCG) did not decrease emphysema severity, but EGCG slightly decreased BALF inflammatory cell numbers and lactate dehydrogenase activity. Inflammation and emphysema persisted after a 17-week recovery period following exposure to CS-250 for 22 weeks. The similarities of this model to the human disease make it promising for studying disease pathogenesis and assessing new therapeutic interventions.

The Epithelial Cell in Lung Health and Emphysema Pathogenesis

Cigarette smoking is the primary cause of the irreversible lung disease emphysema. Historically, inflammatory cells such as macrophages and neutrophils have been studied for their role in emphysema pathology. However, recent studies indicate that the lung epithelium is an active participant in emphysema pathogenesis and plays a critical role in the lung's response to cigarette smoke. Tobacco smoke increases protease production and alters cytokine expression in isolated epithelial cells, suggesting that these cells respond potently even in the absence of a complete inflammatory program. Tobacco smoke also acts as an immunosuppressant, reducing the defense function of airway epithelial cells and enhancing colonization of the lower airways. Thus, the paradigm that emphysema is strictly an inflammatory-cell based disease is shifting to consider the involvement of resident epithelial cells. Here we review the role of epithelial cells in lung development and emphysema. To better understand tobacco-epithelial interactions we performed microarray analyses of RNA from human airway epithelial cells exposed to smoke extract for 24 hours. These studies identified differential regulation of 425 genes involved in diverse biological processes, such as apoptosis, immune function, cell cycle, signal transduction, proliferation, and antioxidants. Some of these genes, including VEGF, glutathione peroxidase, IL-13 receptor, and cytochrome P450, have been previously reported to be altered in the lungs of smokers. Others, such as pirin, cathepsin L, STAT1, and BMP2, are shown here for the first time to have a potential role in smoke-associated injury. These data broaden our understanding of the importance of epithelial cells in lung health and cigarette smoke-induced emphysema.