Cell Kinetics

Cell Kinetics - A Useful Endpoint for Inhalation Toxicology Studies Cells in mitosis are rarely observed in the adult rodent lung. The rate of cell proliferation increases when animals are exposed to agents by inhalation and intratracheal instillation as well as by the blood. Tritiated thymidine and 5-bromo-2'-deoxyuridine (BrdU) incorporation into cells undergoing DNA synthesis are used to assess rates of cell proliferation in the lung. Specifics of the techniques of proper measurement of cell proliferation are important and will be covered in a future paper. It should be noted that few attempts to measure other parameters of the cell cycle such as G1, G2 and M phase have been attempted in the lung. The population of proliferating cells may be epithelial, endothelial, interstitial, PAM (pulmonary alveolar macrophages), or combinations thereof depending upon the exposure agent,and dose and time of exposure. In addition, different areas of the respiratory tract may be affected more than others. For example, the centriacinar (terminal bronchiolar-alveolar) region is most sensitive to ozone. Increases in cell proliferation have generally been considered to be associated with morphological injury as detected with light and electron microscopy. Return to normal levels of proliferation usually corresponds with repair of the lung or cessation of damage. The magnitude of type II cell proliferation in particular correlates with degree of damage to the alveolar region. This correlation was first shown after exposures to oxidant gases such as ozone and NO2. In many instances, cell proliferation increases in the lungs without signs of morphological injury. This phenomenon is often seen when inert or nontoxic particles are inhaled and is related to an influx of inflammatory cells from the blood to the airspaces of the lung. The significance of this transient proliferative response is unclear. It has also been observed after unilateral pneumonectomy , collapse of a lung, and bronchoalveolar lavage. Quite unexpected was the finding that sham exposed control animals in both whole body and nose only inhalation chambers exhibit significant increases in proliferation of all cell types in the lungs. The information obtained from studies of proliferating cells can be useful in determining the mechanisms of action of inhaled agents. The adaption of the lung to ozone as well as oxygen toxicity can be explained with the help of kinetic studies. Much less information is available on cell proliferation during and after exposure to other common air pollutants such as tobacco smoke and diesel exhaust. The paucity of information is probably related to the perceived difficulty in performing cytokinetic studies. Even more helpful in assessing the toxicity of a compound is integration of this information with other changes measured simultaneously in the lungs. These other changes might be in terms of pathology, respiratory mechanics, composition of BAL (bronchoalveolar lavage) fluid, immunology and/or metabolism. These changes will ultimately affect the animal in terms of function. Table 1 is an example of agents known to cause cell proliferation in the lung. Increases in lung cell proliferation may also result in permanent morphological and functional changes in the lungs even after exposure has ceased. Examples include fibrosis, emphysema, hyperplasia, metaplasia, tumor development, and granuloma formation. Unfortuanately cell proliferation studies have not yielded a great deal of information concerning mechanisms of lung tumor development and stem cells of tumors. This may be due to the fact that cancer and precancerous cells do not necessarily have different rates of DNA synthesis compared to normal cells. Other parameters of the cell cycle and cell growth may change such as the growth fraction and rate of cell death. Although it is not impossible to measure these other parameters, it does require a great deal of technical expertise and patience. In summary the adult lung normally shows very little cell proliferation. However, when exposed to various toxic insults, cell proliferation may increase significantly with corresponding morphological signs of injury and repair of the lung. In most cases the lung cells then return to their low steady state rate of cell proliferation. Cell proliferation may be one of the most sensitive indicators of exposure to inhaled agents.