Mitochondrial antioxidants, protection against oxidative stress, and the role of mitochondria in the production of reactive oxygen species

Mitochondria serve as the major source of reactive oxygen species (ROS) production in cells resulting in antioxidant systems and cell signaling pathways that are unique to mitochondria. Thioredoxin-2 (Trx-2) is the mitochondrial member of the thioredoxin superfamily, and acts specifically to reduce the mitochondrial peroxidase, peroxiredoxin-3. It has been proposed that Trx-2 associates with cytochrome c, which functions in mitochondrial respiration and apoptosis. Homozygous Trx-2 deletion in mice is embryonic lethal and it is hypothesized here that Trx-2 lethality is caused by loss of mitochondrial function and oxidative stress. Results of experiments investigating mitochondrial integrity, cell viability, and ROS levels in Trx-2(-/-) mouse embryonic fibroblasts (MEFs), and results from Trx-2 siRNA MEFs, are similar to findings of knockouts in previously reported proteins that function in mitochondrial respiration and support the involvement of Trx-2 in this process. Mitochondrial ROS have also been implicated as major secondary messengers in cell signaling. Results reported here using cancer cells and cancer cells depleted of mitochondrial DNA, which consequently produce few ROS, have indicated that mitochondrial ROS produced in hypoxia are necessary for HRE and ARE activation, and are fundamental in the activation of SP-1 during reoxygenation. However, mitochondrial ROS are not required for HIF-1alpha protein expression in hypoxia, indicating a unique relationship between HIF-1alpha, hypoxia, and mitochondrial ROS.