Reactive oxygen species (ROS) contribute to alveolar cell death in Acute Respiratory Distress Syndrome (ARDS) and we previously demonstrated that NOX1-derived ROS contributed to hyperoxia-induced alveolar cell death in mice. signalling pathways. Thiazovivin In the present study we show that NOX1 is detected in alveolar epithelial cells of ARDS patients in the exudative stage. In addition increased alveolar epithelial cell death and phosphorylated STAT3 are observed in ARDS patients and associated with NOX1 expression. Phosphorylated STAT3 is also correlated with TUNEL staining. We also confirmed that NOX1-dependent STAT3 activation participates to alveolar epithelial cell death. Silencing and acute inhibition of NOX1 in MLE12 led to decreased cell death and cleaved-caspase 3 induced by hyperoxia. Additionally hyperoxia-induced STAT3 phosphorylation is dependent on NOX1 expression and associated with cell death in MLE12 and mice. This study demonstrates that NOX1 is involved in human ARDS pathophysiology and is responsible for the damage occurring in alveolar epithelial cells at least in part via STAT3 signalling pathways. studies have demonstrated that diphenyleneiodonium (DPI) a non-specific inhibitor of NOX enzymes reduces ROS generation in a murine epithelial cell line (MLE12) [9] and in primary pulmonary type II cells [9 10 under hyperoxic condition. Several redox-sensitive signalling pathways including signal transducer and activator of transcription (STAT) PI3K/Akt mitogen-activated protein kinase (MAPK) pathways have been also shown to participate to cell death mediating acute lung injury [7 11 We previously demonstrated that NOX1 contributed to hyperoxic lung damage in part through MAPK activation in mice [7] however the role Thiazovivin of NOX1 in STAT3 signaling-dependent alveolar epithelial cell death was not elucidated in ARDS/ALI. In the present study we first examined whether NOX1 is correlated to epithelial cell death in Acute Respiratory Distress Syndrome and associated with STAT3 signaling. In parallel we confirm the role of STAT3 activation in NOX1-dependent epithelial cell death in hyperoxia by using a murine epithelial cell line and in mice. Methods Control and ARDS patients Human lung biopsies of patient suffering from ARDS (n=10) in the exudative phases and human control lungs (n=10) were obtained by thoracotomy in accordance to an approved protocol by the Institutional Ethical Committee of Geneva (Authorization N° NAC 10-052R). Control lungs were obtained from a pulmonary lobectomy removed for carcinoma. Parenchyma non adjacent to the tumor was used. The exudative phase was defined by the disruption of alveolo-capillary barrier pulmonary edema protein accumulation and inflammatory cell infiltration into EPHB2 the alveolar space. Human immunohistochemistry Paraffin-embedded sections of human lungs fixed with 4% paraformaldehyde were subjected to heat-induced epitope retrieval for 15 min in 0.01 mol/L citrate Thiazovivin buffer (pH 6.0) and endogenous peroxidase was blocked by adding DAKO peroxidase block solution. After blocking in 10% normal goat serum and 1% bovine serum albumin in PBS solution lung sections were stained with an anti-NOX1 polyclonal antibody (1:500; kindly provided by Pr. Lambeth [17] followed by an incubation with a biotinylated goat anti-rabbit Ig (1:100; Vector Laboratories Servion Switzerland) or with an antibody anti-digoxigenin-AP Fab fragments for 30 min at room temperature (1:500; Chemicon Darmstadt Germany) as described by the Thiazovivin manufacturer (ApopTag? Peroxidase In Situ Apoptosis Detection Kit Chemicon Darmstadt Germany) or with an anti-phospho-STAT3 monoclonal antibody (Tyr705 1 Cell Signaling Allschwil Switzerland) anti-prosurfactant C polyclonal antibody (1:1000 Chemicon Darmstadt Germany.) or alternatively with the monoclonal antibody M30 (M30 CytoDEATH Roche Basel Switzerland) for 60 min. Negative controls were obtained by incubating the sections with a biotinylated goat anti-rabbit Ig only (1:100; Vector Laboratories Servion Switzerland) or alternatively with a IgG2a (1:50) in DAKO antibody dilution buffer. The detection of positive Thiazovivin cells was made using Fast Red substrate system (Dako SA Geneva Switzerland) or horseradish peroxidase anti-mouse or rabbit Envision+ system with diaminobenzidine (DAB Dako SA Geneva Switzerland). Sections were then counterstained with cresyl violet and mount with Ultrakitt. Quantification of positive staining was performed using Metamorph analysis software (10 images per subjects 3 subjects per group). Cell culture and hyperoxia experiments Murine lung. Thiazovivin