Apoptosis in heat-induced cell killing: the protective role of hsp-70 and the sensitization effect of the c-myc gene.

We studied heat-induced apoptosis and loss of clonogenicity in Rat-1 fibroblasts, thermotolerant Rat-1 (TT Rat-1), Rat-1 transfected with the human hsp-70 gene (M21) and Rat-1 transfected with the human c-myc proto-oncogene (Rat-1:myc). Relative to Rat-1, TT Rat-1 and M21 cells are heat-resistant, but Rat-1:myc cells are heat-sensitive, in terms of both apoptosis and clonogenic survival. The apoptotic fractions assayed 24 h after a heat treatment of 60 min at 44 degrees C, are about 20% for Rat-1, 7% for TT Rat-1, 10% for M21 and 70% for Rat-1:myc cells, respectively. Most of the apoptotic cells detach from substratum within 1 day of heat treatment and exhibit morphological changes, chromatin condensation and DNA fragmentation. The results of this study suggest that (1) apoptosis is an important mechanism of heat-induced cell killing in some cell lines, (2) apoptosis-mediated cell killing manifests rapidly (relative to clonogenic assay) after a heat treatment, and (3) overexpression of the human hsp-70 gene reduces, whereas expression of the human c-myc proto-oncogene enhances, heat-induced apoptosis. Lastly, the effects of the hsp-70 and c-myc genes on the thermosensitivity of cells are correlated with their modulation of apoptosis.