Redox-cycling of anthracyclines by thioredoxin system: increased superoxide generation and DNA damage

Anthracyclines such as doxorubicin and daunomycin undergo bioreductive activation by redox-cycling, and this is associated with generation of reactive oxygen species. Toxicity of anthracyclines is attributed to DNA intercalation by an anthracycline semiquinone radical that is generated via redox-cycling. Flavoprotein enzymes catalyze the bioreductive activation of anthracyclines. Thioredoxin reductase (TR), which is also a flavoprotein enzyme, participates in bioreductive activation of anthracyclines. In the present study we showed that addition of E. coli thioredoxin (Trx) enhances the rate of superoxide production by E. coli TR in the presence of anthracyclines. The superoxide generated in this redox-cycling process induced DNA damage as determined by an in vitro plasmid DNA damage assay. In addition, Trx-SH enhanced the activity of cytochrome P450 reductase and the redox-cycling of anthracyclines independently of NADPH. Furthermore, when A549 cells were incubated with E. coli Trx followed by doxorubicin treatment, increased levels of ROS generation were observed. Taken together, these results show a novel property of the Trx system in bioreductive activation of anthracyclines.