Exposure to ambient levels of particles emitted by traffic worsens emphysema in mice

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Abstract

Objectives

We investigated effects of chronic exposure (2 months) to ambient levels of particulate matter (PM) on development of protease-induced emphysema and pulmonary remodeling in mice. Methods: Balb/c mice received nasal drop of either papain or normal saline and were kept in two exposure chambers situated in an area with high traffic density. One of them received ambient air and the other had filters for PM. Results: mean concentration of PM10 was 2.68±0.38 and 33.86±2.09 μg/m3, respectively, in the filtered and ambient air chambers (p<0.001). After 2 months of exposure, lungs from papain-treated mice kept in the chamber with ambient air presented greater values of mean linear intercept, an increase in density of collagen fibers in alveolar septa and in expression of 8-isoprostane (p=0.002, p<0.05 and p=0.002, respectively, compared to papain-treated mice kept in the chamber with filtered air). We did not observe significant differences between these two groups in density of macrophages and in amount of cells expressing matrix metalloproteinase-12. There were no significant differences in saline-treated mice kept in the two chambers.

Conclusions

We conclude that exposure to urban levels of PM worsens protease-induced emphysema and increases pulmonary remodeling. We suggest that an increase in oxidative stress induced by PM exposure influences this response. These pulmonary effects of PM were observed only in mice with emphysema.

Introduction

Chronic obstructive pulmonary disease (COPD) is characterized by not fully reversible airflow obstruction that is usually progressive and associated with an abnormal inflammatory response of the lung to noxious particles or gases (Global Initiative for Chronic Lung Disease, 2008). The chronic airflow limitation characteristic of COPD is caused by a mixture of small airway disease and parenchymal destruction (emphysema; Rabe et al., 2007). COPD is associated with enormous health and economic burdens. It affects 10% of the general population and is ranked to become the third commonest cause of death worldwide by 2020 (Barnes, 2007).

The major etiological factor for COPD is chronic oxidative stress as a result of long-term smoking, use of biomass fuels and air pollution exposure (Grievink et al., 2000). Air pollution is considered a risk factor in the development, acceleration, exacerbation and mortality in COPD (Zanobetti et al., 2008; Torres-Duque et al., 2008). Short-term exposure to air pollution has been consistently associated with increased hospital admissions, exacerbations or mortality in patients with COPD (Sunyer, 2001). The effects of long-term, chronic exposure to air pollution on deterioration of lung function and development of COPD have been less explored. In fact, there are few clinical and experimental studies about the effects of particulate matter (PM) exposure on COPD in vivo. A previous study suggested that women living less than 100 m from a busy road have decreased lung function and increased rates of COPD than women living farther away (Viegi et al., 2006).

The extent to which chronic exposure to air pollution is related to development of COPD is of public health and scientific relevance. However, epidemiological long-term studies are difficult to conduct, since there are problems in controlling variables such as changes of exposure during the study, duration of disease, socio-economical factors and the nature of past exposures, among others (Yoshida and Tuder, 2007).

Previous studies with rodents showed an association between air pollution exposure and adverse respiratory effects. Saldiva et al. (1992) observed that rats after 6 months of exposition to urban air pollution developed secretory cell hyperplasia in the airways and ultrastructural cilliary alterations of the epithelium of the airways, suggesting that chronic exposure to urban levels of air pollution may cause respiratory alterations. Lemos et al. (1994) showed that rats submitted to prolonged exposure to low levels of air pollution deteriorated respiratory defenses against infectious agents. Among air pollution components, particulate matter levels have been associated with adverse respiratory health effects. Batalha et al. (2002) observed that short-term exposure of concentrated ambient particles induced vasoconstriction of small pulmonary arteries in normal rats and in rats with chronic bronchitis.

The effects of pollutant exposure in respiratory system were mainly observed in susceptible subgroups, such as children, subjects with chronic respiratory diseases and elderly. (Donaldson et al., 2001). The mechanisms of PM-induced health effects are believed to involve pulmonary inflammation and oxidative stress (Kodavanti et al., 2008; Li et al., 2008).

Emphysema is a major component of COPD, and is characterized by alveolar extracellular matrix destruction, resulting in airspace enlargement with reduction in the alveolar capillary exchange area (Barnes et al., 2003). The proposed pathogenesis for emphysema development involves a combination of inflammation, elastase and matrix metalloprotease imbalance, apoptosis and oxidative stress (Groneberg and Chung, 2004). Protease-induced emphysema produces pathophysiological effects that resemble the human disease (Kawakami et al., 2008). Such experimental models have been widely used to study particle-induced lung disease, and therefore could be an adequate method to study the role of particle-related ambient air pollution in the development of emphysema (Mattson et al., 2008).

The present study was designed to explore the possible role of particles emitted by traffic in aggravating the development of emphysema in mice. We exposed mice, controls or treated with papain, either to filtered or ambient air in inhalation chambers located at a crossroad with high traffic in São Paulo, downtown, for a 2-month period, and analyzed mean alveolar diameter (Lm), the proportion of elastic and collagen fibers, macrophages, MMP-12 expressing cells and 8-isoprostane expression (as a marker of oxidative stress) in the alveolar parenchyma.

Section snippets

Methods

This study was approved by the institutional review board. Six to eight week old male Balb/c mice were used in this study. All animals received care in compliance with the “Principles of Laboratory Animal Care” published by the National Institutes of Health.

Exposure assessment

Mean values of NO2 and PM10 concentration measured during the exposure are presented in Figs. 2(A and B). There was a substantial reduction in the levels of PM10 in the chamber with filtered air when compared to the chamber with ambient air (p<0.001). There was no significant difference in NO2 levels (Fig. 2A), temperature and humidity between the two chambers. The concentrations of NO2 and PM10 in the chamber with ambient air were similar to those measured in the environment. Outdoor mean

Discussion

To our knowledge, this study is the first to demonstrate that chronic exposure of experimental animals to urban levels of traffic-related PM worsens protease-induced emphysema. Mean linear intercept, i.e., the mean diameter of distal airspaces and the total amount of collagen fibers in parenchyma were significantly greater in the lungs of mice that were treated with papain and exposed to ambient particles compared to those mice treated with papain and exposed to filtered air for 2 months (Fig. 3

Acknowledgments

This study was supported by the following Brazilian scientific agencies: Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

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