Activation of Heat-Shock Transcription Factor 1 in Heated Chinese Hamster Ovary Cells Is Dependent on the Cell Cycle and Is Inhibited by Sodium Vanadate

Inducible heat-shock protein 70 (HSP72) is expressed in a cell cycle-specific manner in Chinese hamster ovary (CHO) cells after heating for 15 min at 45.0 degrees C, with the highest level in S-phase cells. Since heat shock induces the transcription of heat-shock proteins through the transactivation of heat-shock elements (HSEs) by heat-shock factor HSF1, we wished to determine whether the cell cycle-specific expression of HSP72 was regulated at the level of transcription. The levels of HSF1 did not vary through the cell cycle, as measured by polyclonal antibodies and flow cytometry. The binding of HSF1 to the heat-shock element was measured with the gel mobility shift assay using cell extracts from Hoechst 33342-labeled heated cells sorted from G1, S and G2/M phases. The HSF1-HSE binding activity was twofold higher in S phase than in G1 or G2/M phase. When CHO cells were exposed to 10 microM sodium vanadate, an inhibitor of tyrosine phosphatase, for 24 h before heat shock, the binding of HSF1 to HSE was reduced by a factor of 2 and the level of HSP72 was greatly reduced. The HSF1 binding to HSE was completely eliminated by using anti-HSF1 antibody in the gel mobility shift assays. Antibodies against HSP73 did not reduce the HSF1-HSE binding activity, but antibodies against HSP40 actually increased the binding activity. These results support the hypothesis that cell cycle-dependent binding of HSF1 to HSE is the cause of the cell cycle-specific expression of HSP72 in heated CHO cells and is regulated by phosphorylation.