Lung cancer continues to be the leading cause of cancer death in the United States. Limited therapeutic success means that prevention strategies may be the most effective intervention for reducing the burden of lung cancer. In a phase II prevention trial, treatment with the oral prostacyclin analogue iloprost resulted in normalization of endobronchial dysplasia (a precursor to squamous carcinoma) in former smokers. About half of former smokers, but not current smokers, had improved endobronchial dysplasia in response to iloprost, suggesting the potential for targeted prevention in specific subjects. Iloprost requires the transmembrane receptor Frizzled 9 (Fzd9), not the prostacyclin receptor (IP), for its preventive activity in vitro. Fzd9 expression is decreased by cigarette smoke and increased by prostacyclin. Restoring Fzd9 expression in NSCLC cell lines inhibits transformed growth, suggesting that Fzd9 has a role in lung epithelium maintenance. Despite its importance in the lung and for iloprost activity, we do not understand Fzd9 regulation or how it interacts with iloprost. In Aim 1, we will induce Fzd9 expression loss in normal lung in vitro and in vivo. We hypothesize that loss of Fzd9 expression leads to abnormal growth in lung epithelial cells and is required for prostacyclin reduced tumor burden in vivo. In Aim 2, we will identify pathways that regulate Fzd9 expression and downstream signaling activity. We hypothesize that Fzd9 expression is controlled by transcription factors and miRNA identified in preliminary screening studies. We also hypothesize, based on preliminary data and in silico modeling, that iloprost directly binds Fzd9 to activate critical prevention signaling. In Aim 3, we will test Fzd9 as a predictive biomarker for response to iloprost. We expect that Fzd9 expression in baseline endobronchial biopsies from the iloprost clinical trial will predict response to iloprost treatment. Overall, these studies will define the role of Fzd9 in the mechanism of iloprost prevention and elucidate regulation of Fzd9. Identification of markers predicting a favorable response to prostacyclin will facilitate personalized prevention to maximize treatment benefit and allow for more intelligent trial design. Our proposal promotes the vital but less common approach to translational research, where clinical data is shared with basic scientists in an effort to understand the mechanisms behind clinical observations and improve future clinical studies. Interdisciplinary teams are key to our translational research. We have scientists and physicians with expertise in molecular and cellular studies, medicinal chemistry, animal models of lung cancer, clinical chemoprevention trials, lung pathology, and biostatistics. This project will have a broad impact as a model for prevention and translational research across disciplines.