Interactive Effects of Cigarette Smoke and Ozone in the Induction of Lung Injury

Deepak K. Bhalla,1

Department of Occupational and Environmental Health Sciences, 627 Shapero Hall, Wayne State University, Detroit, Michigan 48202

ABSTRACT

The article highlighted in this issue is "Short-Term Exposure to Aged and Diluted Sidestream Cigarette Smoke Enhances Ozone-Induced Lung Injury in B6C3F1 Mice" by Mang Yu, Kent E. Pinkerton, and Hanspeter Witschi (pp. 99–106).

Environmental toxicology literature consists of studies that commonly describe the adverse effects of single airborne toxicant atmospheres in humans and animals. A number of studies aimed at investigating the effects of inhaled toxicants have utilized pollutant gases such as ozone, nitrogen dioxide, formaldehyde, and sulfur dioxide, and particles such as titanium dioxide, carbon black, road dust, and other fine and ultrafine particles. Pulmonary and cardiovascular effects have been established by morphological, biochemical, immunological and physiological criteria. Although a great deal of information is now available on the properties, toxicology and mechanisms of action of a number of air pollutants, one aspect of inhalation toxicology that has remained relatively unappreciated until recently is the focus on complex exposures. Animal exposures to cigarette smoke and ozone (O3) constitute one important combination representing a complex exposure model. Ozone, a major component of smog, is produced as a result of photochemical reactions in the air and represents a source for exposure of urban populations and exacerbation of preexisting pulmonary conditions. The adverse effects of cigarette smoke on human health are also widely recognized (Hecht, 1999Go). It is the main etiologic agent in chronic obstructive pulmonary disease and a known human carcinogen. While the risks to human health from active smoking are universally accepted, evidence supporting the risk of involuntary exposure to environmental tobacco smoke (ETS) has accumulated in recent years. Environmental tobacco smoke is now regarded as a human carcinogen and as a risk factor for new cases of asthma and lower respiratory tract infections in children (U.S. EPA, 1992Go). However, the toxicological data base for ETS remains relatively limited. Whether an exposure to ETS in individuals that are also exposed to an air pollutant, such as O3, exacerbates the pollutant effects in the lung is one of the poorly understood issues with important consequences. The paper by Yu and colleagues in this issue of Toxicological Sciences addresses this concern of vital health significance.

It is generally recognized that the properties and potency of individual toxicants may be modified when they are combined in complex atmospheres. Pulmonary effects produced by toxicants are also subject to modification when exposures to multiple toxicants occur in sequence rather than simultaneously as in multicomponent atmospheres. However, the mechanisms of action may not be the same in these two cases. In multicomponent atmospheres individual components often interact to form new reaction products with properties that are different from those of the parent components. The ensuing chemical and physical changes under these conditions result in a modified biological response. In sequential exposures involving more than one toxicant, on the other hand, an injury produced by an initial exposure is likely to be modified by a second insult, not due to the formation of new reaction products, but rather to the possibility that the cells and tissues damaged following the initial insult are prone to further injury by a subsequent insult. Although a case for lung injury following exposure to an inhaled toxicant, with exacerbation by a second insult in the form of inhalation exposure to another toxicant, has been made previously (Bhalla, 1999Go), the paper by Yu and colleagues provides experimental evidence for this type of concerted action in the induction of an intensified response.

These investigators used sidestream cigarette smoke as a surrogate of ETS, which is believed to be the primary source of cigarette smoke exposure of nonsmokers. In mice preexposed to cigarette smoke, a subsequent exposure to O3 produced an exaggerated response as indicated by an increase in bronchoalveolar lavage (BAL) neutrophils and lymphocytes, higher BAL protein levels and an elevation in BRDU labeling of epithelial cells in the centriacinar regions of the alveoli. These markers of inflammation, transmucosal permeability, and cell proliferation in an oxidant-sensitive area of the lung collectively suggest that the selected exposures could target multiple sites for a multifaceted response.

Although the responses observed by Yu and colleagues may appear independent of one another, a mechanistic linkage is not only possible, but could very well explain the overall injury (Figure 1Go). It is possible that gases and particulate components of cigarette smoke could react with cell membranes in the respiratory tract and produce a direct injury through oxidative actions. The damage to the cell membranes following an insult has the potential for altering both structural and functional characteristics of the respiratory tract. Under normal conditions the tight junctions formed by the apposition of adjacent cell membranes obliterate the intercellular spaces, thus providing a barrier to bidirectional transmucosal transport. A functional consequence of structural injury to these tight junctions is an increased mucosal permeability, detected by higher serum protein levels in the BAL. This increased permeability resulting from cigarette smoke exposure has been recognized in humans and laboratory animals for over twenty years (Boucher et al., 1980Go; Jones et al, 1980Go). However, the findings of Yu and colleagues demonstrating a further increase in cigarette smoke effect by O3 raises an added concern related to potential health risks from exposure of cigarette smokers to environmental O3. In addition, it should be understood that under certain circumstances acute exposures can have long-lasting consequences. The brief period during which the epithelial barrier is disrupted has implications in addition to serum transport and lung edema. Previous autoradigraphic studies analyzing transmucosal transport of an intratracheally introduced radiolabeled tracer revealed a dramatic accumulation of the tracer in the subepithelial compartments of the airways following disruption of the epithelial barrier by O3 (Bhalla and Crocker, 1986Go). This observation suggests a rapid transport from the airway lumen to subepithelial regions. However, an accumulation of the tracer in the subepithelial region suggests its slower clearance from that compartment. A similar transport of a particulate toxicant or carcinogenic pollutant in the air, such as one or more ETS components, during a brief period of breach in the epithelial barrier could result in accumulation and extended retention of the toxicant in the subepithelial region, thereby creating a scenario for an extended exposure following an acute injury that is ordinarily reversible with time. It is thus apparent that a compromised barrier can facilitate entry of a particulate toxicant and introduce a risk from continued exposure to the particles now trapped in the subepithelial compartment, even when the exposure to cigarette smoke is discontinued.



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FIG. 1. A simplified schematic of likely lung responses leading to injury following exposure to cigarette smoke and ozone.

 
In addition to damage caused by the direct interaction of a toxicant, such as ETS or O3, with cellular components, inflammatory cells are recognized for their critical role in the development of injury. These cells constitute a major component of pulmonary defenses, and under a normal situation do not present a threat to the host system. However, they can be activated by an appropriate stimulus to release proteolytic enzymes and reactive oxygen species capable of interacting with membrane lipids and proteins, thereby damaging cell membranes and other cellular components. A depletion of protective mechanisms by O3 and an alteration of leukocyte profile are among the possibilities raised by Yu and colleagues to explain the increased susceptibility to O3 in ETS preexposed mice. While these represent perfectly logical explanations, it is also reasonable to speculate that in their study, the inflammatory cells receiving an initial stimulatory signal in the form of cigarette smoke were primed for an amplified response upon a subsequent encounter with O3. The inflammatory cells, upon activation, are known to generate and release a variety of bronchoactive agents and chemotactic mediators. Changes in the levels of eicosanoids, cytokines and chemokines in response to cigarette smoke and O3 exposures have been reported by others (Driscoll et al., 1993Go; Koto et al., 1997Go; Hagiwara et al., 2001Go). Yu and colleagues show an added effect of the dual exposure on the release of TNF-{alpha} by macrophages. While the chemokines serve as a signal for recruitment of additional inflammatory cells from blood circulation, other mediators produced by these cells are capable of causing a wide range of pathophysiological changes, including damage to cell membranes, possible disruption of tight junctions with accompanying increase in mucosal permeability, and injury of airway epithelial cells, followed by cell proliferation. Because of the toxic potential of the products released by inflammatory cells, the contribution of recruited cells to lung injury (amplification of direct deleterious effects of O3 and cigarette smoke) represents an important aspect of the host response, and deserves consideration while defining the overall injury process.

The paper by Yu and colleagues not only attempts to define the injury from a mechanistic perspective, but has implications relevant to the health consequences of exposure to multiple airborne pollutants. The exposure scenario used in their study applies not only to individuals inhaling sidestream smoke and further exposed to oxidant pollutants, such as O3, but it is also relevant to active smokers living in urban areas with high levels of air pollution. Although one may contend that the smoke concentrations in this animal study are considerably higher than those encountered in the air, higher concentrations are generally necessitated by the short-term nature of laboratory studies to simulate lifetime real-world exposure conditions. Despite this criticism, the study serves an important role in moving research in this area towards realization of potential health consequences of exposure to air pollutants, such as O3 in individuals who are also exposed to cigarette smoke. The study raises the possibility that the effects of O3 may be different in individuals exposed to cigarette smoke than in nonsmokers. Although this short-term exposure study may not, by itself, provide an extensive base for imposing stringent regulatory measures for real-world exposures, it certainly provides a stimulus for further research in evaluating a potential threat to smokers' health from exposure to environmental pollutants.

NOTES

1 For correspondence via fax: (313) 577-5589. E-mail: ad6268{at}wayne.edu. Back

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