Role of NOX4 in Modulating Alveolar Type II (AT2) Epithelial Cell Phenotype and Mitochondrial Dynamics in Hyperoxic Acute Lung Injury

ABSTRACT

Introduction: Management of acute respiratory distress including COVID-19 pneumonia involves O2 supplementation, which is lifesaving, but causes severe hyperoxic acute lung injury (HALI). AT2 cells are the most affected cell type in hyperoxia (HO). NADPH oxidase (NOX) is a major source of reactive oxygen species (ROS) in HO. NOX4, the only functionally active NOX present in mitochondria, and primarily produces H2O2 as well as mtROS has been shown to be involved in several human pathologies. Not much is known about NOX4-induced mitochondrial injury in HALI. The current study aims to determine the role of AT2 epithelial cell NOX4 in HALI and the impact of HO on the modulation of mtROS and mitochondrial dynamics in HALI. Methods: Nox4-/-Spc-Cre animals were generated using tamoxifen induction and the knockdown was validated. The Nox4- /-Spc-Cre knockout (KO) and wild type (WT) mice were exposed to room air (NO) or 95% O2 (HO) for 66h to study the structural and functional changes in the lung. Transmission Electron Microscopy (TEM) was used to study the HO-induced changes in mitochondria. Isolated primary AT2 and/ mouse lung epithelial (MLE) cell line was investigated for mtROS, mt dynamics and apoptosis. Mitochondrial injury was assessed in Nox4 WT and Nox4 silenced cells. Results: C57BL/6J WT animals subjected to HO for 66h showed increased expression of NOX4, determining the role of NOX4 in HALI. The H&E staining demonstrated significant HALI in Nox4 WT animals exposed to HO compared to Nox4 KO as determined by increased infiltration of neutrophils, alveolar wall thickening and presence of proteinaceous debris in the alveolar space. Further, increased BAL cell count and protein levels, increased AT2 cell death and elevation of the proinflammatory cytokine IL- 6 and the chemokine KC was seen in WT animals compared to Nox4 KO. Analysis of lung tissues by TEM showed mitochondrial swelling, cristae damage and mitophagy in AT2 cells due to HO. Changes in mt injury markers were also observed. HO-induced NOX4 increase in primary AT2/ MLE-12 cells resulted in increased mtROS production and apoptosis, which was reduced with Nox4 siRNA silencing. Conclusion: This study suggests that the HO induced NOX4 expression in mouse lung, and deletion of Nox4 gene in AT2 cells reduced mtROS production and apoptosis and protected the lungs from severe hyperoxic lung injury. These results suggest NOX4 as a potential target for the treatment of HALI.