The ultimate goal of most toxicological research - extrapolating of results from in vivo and in in vitro studies in animals, tissues and cells to humans - represents a great challenge. Species differences in response often make it difficult to use toxicological data directly for human extrapolation. With respect to particle induced lung injury it has been shown that rats and mice respond very differently to long term inhalation of highly insoluble particles, i.e., the pulmonary inflammatory response was less pronounced in mice, and fibrotic lung lesions and lung tumors were either not found in chronic particle inhalation studies or only to a lesser extent than in rats. Our strategy is to exploit reported differences between rats and mice and to design new studies in order to uncover mechanisms of particle-induced lung injury and facilitate our efforts for extrapolation. This research involves Comparisons of pulmonary response to particles in vivo between Fisher-344 rats and B6C3F1 mice and of responses from in vitro exposures between rat, mouse and human lung cells/tissues. Our comparative studies are based on the hypothesis that the lower fibrotic response in mice to inhaled highly insoluble particles is due to different expression and inducibility of cytokines and metallothionein (MT) in pulmonary target cells and that MT protects lung cells from oxidative damage and subsequent fibroblast proliferative responses. Thus, the emphasis in our proposed studies is to examine differences in dose-response data involving in vivo studies in rats and mice after inhalation and intratracheal instillation of TiO2 particles (2 distinct particle sizes, i.e., ultrafine (approximately 20 nm) and larger- sized (approximately 250 nm) and SiO2 particles (cristobalite, approximately 0.8 micromoles). Exposure-Dose-Response relationships will be established in both species focusing on particle retention kinetics, inflammatory and cell proliferative responses correlated with cytokine and growth factor expression and MT induction. Fibrotic events will be evaluated by histology and quantification of collagen deposition. The role of the alveolar macrophage (AM) will be assessed using AM-depleted rats and mice and by examining AM function isolated after in vivo exposure. Complementary in vitro exposure studies using lung cells and lung tissue culture will be performed to determine particle dose-response (cytokines and MT) relationships for AM at the cellular level, cell-specific inducibility of MT in AM and type II cells and cytokine expression, cell proliferative responses and MT induction in lung tissue culture. The significance of MT will be further evaluated in in vivo and in vitro studies by inducing this protein by exposure to zinc before or in combination with particle exposure. By investigating and characterizing species specific mechanically based parameters we will not only gain a better understanding of mechanisms of particle-induced lung injury, but additionally, these studies will strengthen the basis for mechanistically- oriented extrapolation of results observed in either rats or mice to humans.
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