Thiol reducing reagents inhibit the heat shock response. Involvement of a redox mechanism in the heat shock signal transduction pathway.

We evaluated the effects of thiol-reducing agents on the heat shock response in human and rodent cells in culture. Using HeLa cells as an example, we demonstrated that dithiothreitol (DTT,2mM) inhibited the heat (42 degrees C) induced increase in the synthesis of heat shock proteins (HSPs), abundance of mRNA of hsp 70, hsp 70 gene promoter activity, and the heat shock factor (HSF) DNA binding activity. This effect of DTT was specific and attributable to its reducing activity; oxidized DTT was ineffective, and other thiol reducing compounds had the same effect as DTT. Time course and dose-response studies showed that DTT significantly inhibited the heat shock induction of heat shock element binding activity with no preincubation and that 0.6 and 1-2 mM DTT gave half-maximal and maximal inhibition, respectively. The effect of DTT was reversible; removal of the DTT-containing medium prior to heat shock rendered the cells fully responsive. Analysis of the effects of DTT on the regulation and function of HSF suggests that DTT blocked an early and important step in the activation process without having a direct effect on the HSF protein. Thus, DTT inhibited the heat-induced trimerization, phosphorylation, and nuclear translocation of HSF and was also effective against a number of other reagents that are known to activate HSF. On the other hand, DTT did not block the response induced by heat shock at 45 degrees C, and in vitro addition of DTT failed to modulate the DNA binding activity of activated HSF present in cell extracts, suggesting that the HSF protein itself is unlikely to be a direct target of action of DTT. These results, together with the observation that activation of HSF DNA binding activity was attenuated under an anoxic condition and that hydrogen peroxide mimicked the effects of heat shock, suggest the involvement of a redox mechanism as an early and important step in the heat shock signal transduction pathway.