Thioredoxin nuclear translocation and interaction with redox factor-1 activates the activator protein-1 transcription factor in response to ionizing radiation

Thioredoxin (TRX) is a cytoplasmic, redox-sensitive signaling factor believed to participate in the regulation of nuclear transcription factors mediating cellular responses to environmental stress. Activation of the activator protein (AP)-1 transcription factor is thought to be mediated in part by redox-sensitive interactions between the nuclear signaling protein redox factor-1 (Ref-1) and TRX. In this study, the role of TRX and Ref-1 in the activation of the AP-1 complex was examined in HeLa and Jurkat cell lines exposed to ionizing radiation (IR). After exposure to IR, nuclear levels of immunoreactive TRX increased, accompanied by an increase in AP-1 DNA binding activity. It was shown that a physical interaction between Ref-1 and TRX occurs within the nucleus and is enhanced after exposure to IR. Furthermore, TRX immunoprecipitated from irradiated cells was capable of activating AP-1 DNA binding activity in nonirradiated nuclear extracts. In addition, immunodepletion of Ref-1 from nuclear extracts demonstrated that the increase in AP-I DNA binding activity after IR was also dependent upon the presence of Ref-1 from irradiated cells. Finally, the ability of both TRX and Ref-1 from irradiated cells to stimulate AP-1 DNA binding in nonirradiated nuclear extracts was abolished by chemical oxidation and restored by chemical reduction. These results indicate that, in response to IR, TRX and Ref-1 undergo changes in redox state that contribute to the activation of AP-1 DNA binding activity. These experiments suggest that a redox-sensitive signaling pathway leading from TRX to Ref-1 to the AP-1 complex participates in the up-regulation of DNA binding activity in response to ionizing radiation.