Abstract In this competitive renewal of my first R01, we propose new experiments to test the hypothesis that presqualene diphosphate levels in cells are regulated by polyisoprenyl diphosphate phosphatase 1, which serves as a target for select counter-regulatory lipid mediators that promote resolution of acute lung inflammation and injury. Local airway trauma secondary to accidental aspiration of gastric acid is a frequent cause of acute lung injury and acute respiratory distress syndrome with neutrophil (PMN) - predominant inflammation in the lung. Despite a robust inflammatory response, bacterial pneumonia is a frequent complication. To date, anti- inflammatory agents have not proven effective in reducing the associated critical illness, mortality and prolonged disability in ARDS survivors. Novel therapeutic approaches are needed. During the last funding cycle, we made significant scientific progress towards all of our original specific aims and established polyisoprenyl phosphate (PIPP) remodeling as a new counter-regulatory signaling pathway in PMN. We determined structure-activity relationships for presqualene diphosphate (PSDP) inhibition of phospholipase D, identified endogenous PSDP as a directly inhibitor of phosphatidyl inositol 3- kinase in the regulation of cell activation and uncovered polyisoprenyl diphosphate phosphatase 1 (PDP1) as the first PSDP phosphatase capable of catalyzing the conversion of PSDP to its inactive monophosphate form (PSMP) that is 2 log orders less potent than PSDP as an inhibitor. In addition, we prepared new bioactive PSDP mimetics with in vivo protective actions in acid-initiated acute lung injury. Lipoxins are arachidonic acid-derived autacoids that also display protective actions in acute lung injury and regulate leukocyte actions via inhibition of PSDP remodeling, suggesting a pivotal counter-regulatory signaling role for PDP1 in cell activation. In extrapulmonary tissues, intriguing tissue-protective actions have recently also been assigned to three new classes of autacoid lipid mediators derived from eicosapentaenoic and docosahexaenoic acids. To test our hypothesis, we propose three specific aims: Elucidate factors responsible for cellular PDP1 activation, Define the influence of PDP1-based counter-regulatory lipid mediators on select human PMN and airway epithelial functional responses, and Establish the regulation of acute lung injury resolution by PDP1-based lipid mediators. These specific aims will enable us to (i) uncover new molecular insights into lipid-derived signaling pathways during homeostatic loss and recovery in the lung; and (ii) design novel therapeutic strategies using these lipid signals as templates. Project Narrative Acute lung injury/acute respiratory distress syndrome (ALI/ARDS) is a serious and explosive illness with substantial morbidity and mortality that affects as many as 75/100,000 population. No matter the cause, once ALI/ARDS is established the disease has no specific treatment. In this proposal, we seek to develop a better understanding of the biological processes invoked after airway injury that naturally promote resolution in order to develop new, more specific therapeutic approaches to prevent the development or lessen the severity of ARDS.