Redox shuttle of cytosolic Thioredoxin to mitochondria protects against hyperoxia-mediated alteration of mitochondrial structure and dysfunction

Cytosolic thioredoxin (Trx) is a critical redox protein that converts protein disulfides to thiols via catalytic activity of thioredoxin reductase-1 (TrxR1) and NADPH. Thioredoxin-2 (Trx2) is a mitochondria-localized isoform. It is generally believed that Trx and Trx2 perform similar functions within the cytosol and mitochondria respectively. Here, we demonstrate that cytosolic Trx shuttles into mitochondria in the presence of normal levels of Trx2 in physiological state and higher levels of Trx translocate to mitochondria in oxidative stress conditions such as exposure to high concentrations of oxygen. This shuttle is required to maintain mitochondrial structure and function during physiological and oxidative stress conditions. Further, reduced Trx (Trx-SH) shuttle into mitochondria to protect against the downregulation of several mitochondrially coded genes and proteins of respiratory chain complexes in oxidative stress. Translocation of Trx occurs only in the reduced state as oxidized or cysteine mutant Trx is unable to translocate to the mitochondria. Accumulation of mitochondrial DNA damage product 8-Oxo-dG in hyperoxia is decreased in the presence of higher levels of cytosolic Trx within the mitochondrion. Collectively, our data demonstrate that shuttling of reduced cytosolic Trx into mitochondria protects against mitochondrial DNA damage, decreased gene and protein expression of respiratory chain complexes and mitochondrial dysfunction resulting in restoration of their native function and cell survival in physiological and oxidative stress conditions.