Attenuation of the heat shock response in HeLa cells is mediated by the release of bound heat shock transcription factor and is modulated by changes in growth and in heat shock temperatures

When HeLa S3 cells are subjected to a continuous 42°C heat shock, activation of heat shock transcription factor (HSF) and transcriptional activation of the heat shock genes Asp 70, hsp89α, and hsp60 is transient, peaking at 40-60 min of heat shock, and then attenuating. We have used in vivo genomic footprinting to demonstrate that attenuation of hsp70 transcription is mediated by release of bound HSF from the heat shock element (HSE) of the hsp70 gene promoter. Release of bound HSF in vivo occurs at a higher rate than would be predicted from in vitro measurements of dissociation. Attenuation of HSF activation and heat shock gene transcription occurs only when mild heat shock temperatures are employed (42°C); increasing the heat shock temperature by 1°C elicits a much higher level of activation, which does not attenuate during a 4-hr heat shock. Surprisingly, altering the temperature at which cells are grown prior to heat shock modulates the magnitude and temporal pattern of the response to a given heat shock temperature. This finding suggests that HSF does not sense temperature directly but, instead, may be responsive to the magnitude of the difference between growth and heat shock temperatures.