RNA Sequencing Identifies Peroxisome Proliferator-Activated Receptor-Gamma Upregulation as an Immunomodulator in a 'Two-Hit' Mouse Model of Trauma Followed by Acute Lung Injury

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American Journal of Respiratory and Critical Care Medicine



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Rationale: Trauma and hemorrhagic shock (HS) are the most common causes of mortality in young adults. 10-30% of patients with trauma/HS develop acute respiratory distress syndrome (ARDS), which can increase mortality. A ‘two-hit’ model has been proposed to explain the pathophysiology of ARDS following trauma/HS - the traumatic injury acting to ‘prime’ the body for a subsequent infectious or inflammatory insult (the second hit) that results in ARDS. However, the immunologic basis for such a model has not been well studied. Using RNA sequencing, we studied the effect of prior trauma on the inflammatory response to intratracheal lipopolysaccharide (LPS) in a mouse model with a hypothesis-neutral approach. Methods: 6-8-week-old, male C57Bl/6 mice were exposed to hemorrhagic shock + cecectomy (polytrauma) or sham surgery. 24h later the mice were administered intratracheal LPS (5 mg/kg) or an equal volume of saline, creating 4 groups of animals (n=4/group). Mice were euthanized after another 24h. Total RNA was prepared from the left lung and sequenced. Differentially expressed transcripts were identified with CuffDiff and enriched pathways associated with this differential expression were determined using Ingenuity Pathway Analysis (IPA). Expression levels among groups for specific transcripts of interest were confirmed using RT-qPCR. Results: Exposure to polytrauma prior to an intratracheal LPS challenge substantially decreased pro-inflammatory genes and pathways compared to animals exposed to sham surgery and LPS. These results were confirmed by measuring mRNA levels of multiple inflammatory cytokines (e.g. IL6, Fig 1B, top). IPA analysis identified the Peroxisome Proliferator-Activated Receptor (PPAR) and Liver X Receptor/Retinoid X Receptor (LXR, RXR) pathways to be altered inversely compared to inflammatory pathways (Fig 1A). RT-qPCR of mouse lungs detected a significant increase in PPAR-γ in mice exposed to polytrauma alone compared to LPS exposed groups (Fig 1B, bottom). These data show that PPAR-γ is significantly induced in murine lungs after trauma. PPAR-γ is known to exert immunosuppressive effects by forming heterodimers with RXR. Our data therefore suggest that trauma-induced activation of PPAR-γ signaling may be responsible for the blunted pro-inflammatory response to LPS after exposure to trauma.


Diseases | Medicine and Health Sciences



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