Heat shock factor 1-mediated thermotolerance prevents cell death and results in G(2)/M cell cycle arrest

Mammalian cells respond to environmental stress by activating heat shock tr anscription factors (eg, Hsf1) that regulate increased synthesis of heat sh ock proteins (Hsps). Hsps prevent the disruption of normal cellular mitosis , meiosis, or differentiation by environmental stressors. To further charac terize this stress response, transformed wildtype Hsf1(+/+), and mutant Hsf 1(-/-) mouse embryonic fibroblasts (MEFs) were exposed to (1) lethal heat ( 45 degreesC, 60 minutes), (2) conditioning heat (43 degreesC, 30 minutes), or (3) conditioning followed by lethal heat. Western blot analysis demonstr ated that only Hsf 1(+/+) MEFs expressed inducible Hsp70s and Hsp25 followi ng conditioning or conditioning and lethal heat. Exposure of either Hsf1(+/ +) or Hsf1(-/-) MEFs to lethal heat resulted in cell death. However, if con ditioning heat was applied 6 hours before lethal heat, more than 85% of Hsf 1(+/+) MEFs survived, and cells in G(2)/M transiently increased 3-fold. In contrast, conditioned Hsf1(-/-) MEFs neither survived lethal heat nor exhib ited this G(2)/M accumulation. Coinfection with adenoviral Hsp70 and Hsp25 constructs did not fully recreate thermotolerance in either Hsf1 (-/-) or H sf 1(-/-) MEFs, indicating other Hsf1-mediated gene expression is required for complete thermotolerance. These results demonstrate the necessity of Hs f1-mediated gene expression for thermotolerance and the involvement of cell cycle regulation, particularly the G(2)/M transition, in this thermotolera nt response.