Activation of heat shock factor 1 DNA binding precedes stress-induced serine phosphorylation. Evidence for a multistep pathway of regulation.

Exposure of mammalian cells in culture to the anti-inflammatory drugs sodium salicylate or indomethacin results in activation of heat shock factor 1 (HSF1) DNA binding activity. We have previously shown that the drug-induced HSF1 becomes associated with the heat shock elements of the hsp70 promoter, yet transcription of the hsp70 gene is not induced (Jurivich, D. A., Sistonen, L., Kroes, R. A., and Morimoto, R. I. (1992) Science 255, 1243-1245). In this study, we have examined the basis for uncoupling the heat shock transcriptional response. Comparison of heat shock and drug-induced forms of HSF1 has revealed that the transcriptionally inert drug-induced HSF1 is constitutively but not inducibly serine-phosphorylated, whereas heat shock-induced HSF1 is both constitutively and inducibly serine-phosphorylated. The transcriptionally inert intermediate represented by drug-induced HSF1 can be converted to the transcriptionally active state by a subsequent exposure to heat shock. The only detectable change in HSF1 is the acquisition of inducible serine phosphorylation. These data reveal that acquisition of the trimeric DNA binding state of HSF1 is independent of and precedes inducible phosphorylation and furthermore that inducible phosphorylation correlates with transcriptional activation.