Redox Homeostasis is Disturbed by Redox Cycling between Reactive Cysteines of Thioredoxin 1 and 9,10-Phenanthrenequinone, an Atmospheric Electron Acceptor.

9,10-Phenanthrenequinone (9,10-PQ) is a toxicant in diesel exhaust particles and airborne particulate matter ≤2.5 μm in diameter. It is an efficient electron acceptor that readily reacts with dithiol compounds in vitro , resulting in the oxidation of thiol groups and concomitant generation of reactive oxygen species (ROS). However, it remains to be elucidated whether 9,10-PQ interacts with proximal protein dithiols. In the present study, we used thioredoxin 1 (Trx1) as a model of proteins with reactive proximal cysteines and examined whether it reacts with 9,10-PQ in cells and tissues, thereby affecting its catalytic activity and thiol status. Intratracheal injection of 9,10-PQ into mice resulted in protein oxidation and diminished Trx activity in the lungs. Using recombinant wild-type and C32S/C35S Trx1, we found that Cys32 and Cys35 selectively serve as electron donor sites for redox reactions with 9,10-PQ that lead to substantial inhibition of Trx activity. Addition of dithiothreitol restored the Trx activity inhibited by 9,10-PQ. Exposure of cultured cells to 9,10-PQ caused intracellular reactive oxygen species generation that led to protein oxidation, Trx1 dimerization, p38 phosphorylation, and apoptotic cell death. Overexpression of Trx1 blocked these 9,10-PQ-mediated events. These results suggest that the interaction of the reactive cysteines of Trx1 with 9,10-PQ causes oxidative stress, leading to disruption of redox homeostasis.