Thermotolerant cells show an attenuated expression of Hsp70 after heat shock.

Expression of heat shock proteins (hsps) results in the protection of cells from subsequent stresses. However, hsps are also toxic when present within cells for a prolonged time period. Thus, the expression of hsps should be tightly regulated. In the present study, the expression of Hsp70 after heat shock was compared between thermotolerant cells, which contain a large concentration of Hsp70, and nonthermotolerant cells (naive). Accumulation of Hsp70, assessed by Western blotting, was negligible when thermotolerant cells were heat-shocked a second time. Hsp70 transcription was similar between thermotolerant and naive cells during heat shock. However, Hsp70 transcription was attenuated more rapidly in thermotolerant than naive cells immediately upon return to non-heat shock conditions. In addition, Hsp70 mRNA stability was reduced in thermotolerant cells as compared with naive cells following the stress. New synthesis of Hsp70 and the efficiency of Hsp70 mRNA translation were similar between thermotolerant and naive cells during the post-stress period. These results suggest that thermotolerant cells limit Hsp70 expression by transcriptional and pretranslational mechanisms, perhaps to avoid the potential cytotoxic effect of these proteins.