DESCRIPTION (Adapted from applicant's abstract and specific aims): The lung is a major target organ for oxidant injury during exposure to hyperoxia. Many pulmonary diseases (e.g. adult respiratory distress syndrome [ARDS], emphysema) require supplemental oxygen therapy to maintain lung function which further increases the oxidant burden of the lung. Damaging effects of hyperoxia are mediated by reactive oxygen species (ROS) including superoxide and hydroxyl radicals, and hydrogen peroxide, which are generated by the incomplete reduction of oxygen. These toxic ROS can damage cellular constituents such as nucleic acids, proteins and lipids. The cellular and molecular responses of the lung to hyperoxia involve increased expression of antioxidant enzymes and stress-response genes, including the stress-inducible gene heme oxygenase-1 (HO-1). The mechanism(s) involved in the regulation of HO-1 expression after hyperoxic stress and the functional significance of HO-1 induction after hyperoxia is largely unknown. This application proposes to examine the molecular regulation of HO-1 gene expression after hyperoxia and delineate the functional significance of HO-1 induction after hyperoxia by addressing the following specific aims: 1) to determine the transcriptional regulation of HO-1 gene induction by delineating the critical cis-acting elements responsible for mediating the induction of HO-1 gene after hyperoxia; 2) to determine the role of activator protein-1 (AP-1) family of transcription factors in the induction of HO-1 gene exposure by analyzing the expression, activation and modulation of AP-1 activity after hyperoxia; 3) to determine the upstream signalling pathways involved in the activation of AP-1 and induction of the HO-1 gene after hyperoxia; and 4) to determine the functional significance of HO-1 induction in vitro and in vivo after hyperoxia.