Heat shock and the activation of AP-1 and inhibition of NF-κB DNA-binding activity:: possible role of intracellular redox status

The early response genes comprising the AP-1 and NF-kappaB transcription factors are induced by environmental stress and thought to modulate responses to injury processes through the induction of target genes. Exposure to heat and ionizing radiation (IR) has been shown to affect signalling machinery involved in AP-1 and NF-kappaB activation. Furthermore, regulation of the signalling pathways leading to the activation of these transcription factors has been linked to changes in intracellular oxidation/reduction (redox) reactions. The hypothesis is proposed that exposure to thermal stress and/or IR might alter metabolic processes impacting upon cellular redox state and thereby modify the activity of redox-sensitive transcription factors such as AP-1 and NF-kappaB. Gel electromobility shift assays (EMSA) demonstrated that heat shock-induced AP-1 DNA-binding activity but inhibited IR-induced activation of NF-kappaB. A time course showed that activation of the AP-1 complex occurs between 4 and 5 h following thermal stress, and inhibition of IR-induced NF-kappaB activation also occurs during this time interval. Using a redox-sensitive fluorescent probe [5-(and -6)-carboxy-2',7'-dichlorodihydrofluorescein diacetate], a shift to 40% less intracellular dye oxidation was observed in HeLa cells 0-4 h post-heat shock (45degreesC, 15 min) relative to cells held at 37degreesC. This was followed by a shift to greater dye oxidation between 4 and 12 h after treatment (about 1.8-fold) that returned to control levels by 24 h post-heating. These results show changes in DNA-binding activity closely paralleled apparent heat-induced changes in the intracellular redox state. Taken together, these results provide correlative evidence for disruption of redox-sensitive IR-induced signalling pathways by heat shock and support the hypothesis that this mechanism might play a role in heat-induced alterations in radiation response.