Our hypothesis is that tobacco smoke (TS) exposure (TSE) promotes apoptosis of airway epithelial cells through a newly identified regulatory pathway that involves induction of plasminogen activator inhibitor (PAI) -1 through posttranscriptional stabilization of PAI-1 mRNA by p53. This hypothesis is supported by our preliminary data and extends a mature body of literature that implicates epithelial cell injury and the participation of the fibrinolytic pathway in the pathogenesis of a wide range of lung injuries, including COPD and airway remodeling induced by TSE. While prior studies implicate p53 protein in airway epithelial cell damage and strongly suggest that the PAI-1 plays a key role in the pathogenesis of increased airway reactivity, the role of PAI-1 in TSE-induced airway injury remains unclear at the present time. Our objectives are to determine how PAI-1 expression is controlled by TSE-induced p53 and elucidate the mechanism by which this pathway affects airway epithelial cell apoptosis. We will then block TSE-mediated induction of PAI-1 to reverse airway and alveolar epithelial cell apoptosis due to TSE. We will apply both in vitro and in vivo approaches to achieve these objectives. We will determine the responses of primary cultures of human small airway epithelial or alveolar type (AT) II cells to TSE in vitro and test the specificity of the responses in parallel studies in which we will test the responses of primary ATII cells isolated from mouse lungs. We will next determine the effects of TSE in WT, p53-/- and PAI-1-/- mice to elucidate the effect of p53-mediated expression of PAI-1 on lung epithelial cell apoptosis in vivo. Our specific aims are: 1. To determine the mechanism by which p53-induced PAI-1, and inhibition of PAI-1 expression, modulate airway and alveolar epithelial cell apoptosis in vitro. 2. To determine how p53-induced PAI-1 and inhibition of PAI-1 expression contribute to LEC apoptosis during TSE-induced lung injury in vivo. These studies address a critical gap in current knowledge about how the fibrinolytic system regulates TSE-induced airway damage via interaction of PAI-1 mRNA with p53. This project extends the ongoing translational work of our laboratory to a clinically important new area. We will deploy our expertise in the area of posttranscriptional mechanisms to now elucidate how p53-mediated PAI-1 expression by the airway epithelium regulates epithelial apoptosis that characterizes airway remodeling induced by TSE. These studies will accelerate the development of novel therapeutic interventions to mitigate lung epithelial injury. PUBLIC HEALTH RELEVANCE: Tobacco smoke exposure (TSE) damages the airway lining otherwise called the lung epithelium. TSE-induced damage to the lining of the lungs characterizes chronic obstructive lung disease (COPD) which is the fourth leading cause of death in the United States. Our findings and those of the literature suggest that TSE-induced airway injury involves programmed airway epithelial cell death, airway inflammation and abnormal fibrin turnover. In this project, we will determine how p53, a protein causing epithelial cell death, regulates PAI-1, an inhibitor of the blood clot dissolution system, to influence the viability of the airway epithelium. These studies will enable us to understand the role of newly identified interactions between p53 and PAI-1 in the pathogenesis of lung epithelial cell injury and airway remodeling and will accelerate the development of new strategies to protect the airways against TSE-mediated damage.