The response of alveolar type II epithelial cells to oxidative stress from hyperoxia

Supplemental oxygen therapy is used to treat premature infants and adults with acute respiratory distress. While necessary for maintaining tissue oxygen levels, exposure to high concentrations of oxygen increases the formation of reactive oxygen species (ROS) in pulmonary cells resulting in lipid and protein oxidation, enzyme inactivation and oxidative DNA damage. Hyperoxia injures and kills microvascular endothelial cells and type I alveolar epithelial cells, while type II cells remain morphologically intact and relatively unaffected. In fact, to repair the damaged epithelium, type II cells also proliferate and differentiate into type I cells during recovery. While it has been shown in vitro that hyperoxia injures and kills type II epithelial cells, it remained unclear if type II cells are protected in vivo from hyperoxia induced ROS or if they accumulate damage but are resistant to cell death. The objective of this thesis was to determine whether hyperoxia induces DNA damage in vivo in type II alveolar epithelial cells and to investigate potential protective mechanisms for type II cell resistance to death during exposure. A transgenic mouse model was established and characterized to allow rapid identification and isolation of type II cells from the lungs of mice. Next, oxidative DNA lesions and DNA strand breaks were identified and characterized in vivo and ex vivo in type II cells isolated from normoxic or hyperoxic mice. Type II cells accumulate DNA damage in vivo although analysis of viability and apoptosis confirmed that these cells do not die during exposure. Finally cell-specific expression of growth arrest and DNA damage inducible protein Gadd45a was determined not to protect airway and type II alveolar epithelial cells from oxidative stress during hyperoxia. Taken together, the data presented in this thesis establish a novel transgenic mouse model for in vivo studies of hyperoxia in type II cells. Furthermore, these studies demonstrate that, although viable and proliferative, type II cells accumulate DNA damage and Gadd45a is not a critical molecule in the relative resistance of these cells to hyperoxia. Finally, the studies presented in this thesis provide a rational basis for the continued investigation of type II cells as a progenitor cell population in the lung with increased resistance to oxidative stress.