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A7536 - Janus-Headed Nature of Succinate in Acute Lung Injury: Alveolar Type II Specific Deletion of Succinate Dehydrogenase A Is Protective in Acute Lung Injury
Author Block: E. Coit1, N. Burns2, R. M. Tuder3, H. Eltzschig4, C. U. Vohwinkel5; 1Cardiovascular Pulmonary Research, University of Colorado Anschutz Medical Campus, Aurora, CO, United States, 2Pediatric Critical Care, University of Colordao Denver, Aurora, CO, United States, 3Pulmonary Dept, Univ of Colorodo Denver Sch of Med, Aurora, CO, United States, 4Dept. of Anesthesiology, Univ of Texas Medical Sch., Houston, TX, United States, 5Pediatric Critical Care, Univ of Colorado Denver, Aurora, CO, United States.
Rationale: Acute lung injury (ALI) is an inflammatory lung disease, which manifests itself in patients as acute respiratory distress syndrome (ARDS). Presently, specific therapeutic approaches for ARDS are essentially unknown. Alveolar epithelial cells are known for their susceptibility to injurious insults that can lead to ALI. Previous studies have shown that cyclic stretch injury inhibits succinate dehydrogenase (SDH) and leads to HIF1A dependent optimization of carbohydrate metabolism in the alveolar epithelium. This response was protective in acute lung injury. Immunoresponsive gene 1 (Irg1) is highly expressed in mammalian macrophages during inflammation and inhibits SDH. We hypothesize, that in AT II cells induction of Irg1 during ALI inhibits SDH resulting in subsequent increases of succinate and stabilization of HIF1A. Methods: Type II alveolar epithelial (AT II) cell specific SDHA knockout mice (SPC-ER Cre SDHA loxp/loxp) were exposed to 4 hours of mechanical ventilator-induced lung injury (VILI). Control animals were ventilated with low peak inspiratory pressure. Protein content of bronchoalveolar lavage (BAL) fluid was analyzed with Bradford assay. Histology was obtained to determine extent of lung injury. Primary AT II cells were isolated using antibody selection. Protein expression in nuclear and cytoplasmic fractions was determined with Western Blot. Cytokine expression was determined with qPCR. For in vitro studies cells were subjected to cyclic mechanical stretch. MLE-12 cells, an alveolar epithelial cell line, was utilized to generate stable Sdha -/- and Hif1a -/- cells. Results: Irg-1 was induced in alveolar epithelial cells in response to VILI and succinate was increased. SPC-ER Cre SDHA loxp/loxp mice were protected from VILI. AT II cells incubated with dimethylsuccinate showed attenuated inflammation in stretch injury. However, succinate was found to be a relatively weak activator of HIF1A compared to hypoxia. This was in line with our findings that SPC-ER Cre Sdha loxp/loxp mice only showed modest induction of HIF1A and glycolysis in AT II cells in response to VILI. Therefore the protective effect of SDHA inhibition is likely mediated by factors other than solely HIF1A stabilization. Conclusions: We have shown that AT II specific inhibition of Sdha via Irg1 is protective in ALI. However, this protective mechanism is only partially mediated through HIF1A induced optimization of glycolysis. Further investigation of the downstream effects of Irg1 is essential to elucidate the mechanism for attenuation of inflammation in ALI. Such downstream effects include the induction of Nrf2 and the effect of SDH inhibition on intermediates of the Krebs cycle.