PROJECT SUMMARY/ABSTRACT Idiopathic Pulmonary Fibrosis (IPF) is a lethal age-related disease characterized by chronic, progressive, and irreversible fibrosis. Cellular senescence has been widely implicated in the pathogenesis of IPF. However, the mechanism promoting senescence in IPF, especially at the post-transcriptional level, is poorly understood. The current proposal will address this knowledge gap by elucidating the role of alternative polyadenylation (APA) in senescent fibroblasts and its contribution to pulmonary fibrosis. Nudix Hydrolase 21 (Nudt21, also known as CFIm25) is an RNA binding protein playing an important role in APA. We recently found that NDUT21 is downregulated in aging and fibrotic lungs as well as in senescent fibroblasts. Nudt21 knockdown in normal lung fibroblasts induces STAT3 phosphorylation and the expression of many senescence-associated secretory phenotype (SASP) factors. Importantly, fibroblast Nudt21 depletion aggravated bleomycin-induced pulmonary fibrosis in mice, whereas NUDT21 rescue led to attenuated fibrosis and SASP release. These findings highlight NUDT21 downregulation as a novel modifiable factor for aging- related IPF and provide a strong foundation to study the role of NUDT21-mediated-APA in lung fibrosis during aging. Based on these extensive preliminary findings, we hypothesize that miRNA-mediated NUDT21 downregulation in senescent fibroblasts promotes pulmonary fibrosis during aging, through APA regulation of STAT3 signaling and SASP induction. We will test this hypothesis in following specific aims: 1). Evaluate the role of NUDT21 as an important mediator in age-associated lung fibrosis; 2) Determine the upstream mechanism for NUDT21 depletion during aging; 3) Define the role of targeting NUDT21 downstream components in pulmonary fibrosis; and 4) Molecular investigation of the miRNA/NUDT21/STAT3 axis in human IPF lungs. These proposed studies are significant because they will elucidate a novel mechanism for enhanced expression of SASP proteins in senescent fibroblasts that could contribute to the pathogenesis of IPF. Innovations include demonstrating the miRNA/NUDT21/STAT3/SASP axis as a novel link between aging and IPF pathogenesis, and new reagents and methodologies including Nudt21 knockout mice, primary lungs and fibroblasts from aged/IPF subjects, and innovative lung functional studies in animal models. Ultimately, results from these studies will facilitate development of novel therapeutic strategies against this devastating age-related disease.