The heat-shock (HS) response is a ubiquitous cellular response to stress, involving the transcriptional activation of HS genes. Reactive oxygen species (ROS) have been shown to regulate the activity of a number of transcription factors. We investigated the redox regulation of the stress response and report here that in the human pre-monocytic line U937 cells, H2O2 induced a concentration-dependent transactivation and DNA-binding activity of heat-shock factor-1 (HSF-1). DNA-binding activity was, however, lower with H2O2 than with HS. We thus hypothesized a dual regulation of HSF by oxidants. We found that oxidizing agents, such as H2O2 and diamide, as well as alkylating agents, such as iodoacetic acid, abolished, in vitro, the HSF-DNA-binding activity induced by HS in vivo. The effects of H2O2in vitro were reversed by the sulphydryl reducing agent dithiothreitol and the endogenous reductor thioredoxin (TRX), while the effects of iodoacetic acid were irreversible. In addition, TRX also restored the DNA-binding activity of HSF oxidized in vivo, while it was found to be itself induced in vivo by both HS and H2O2. Thus, H2O2 exerts dual effects on the activation and the DNA-binding activity of HSF: on the one hand, H2O2 favours the nuclear translocation of HSF, while on the other, it alters HSF-DNA-binding activity, most likely by oxidizing critical cysteine residues within the DNA-binding domain. HSF thus belongs to the group of ROS-modulated transcription factors. We propose that the time required for TRX induction, which may restore the DNA-binding activity of oxidized HSF, provides an explanation for the delay in heat-shock protein synthesis upon exposure of cells to ROS.

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