Cellular mechanisms associated with the lack of chronic thermotolerance expression in HeLa S3 cells

Chronic thermotolerance is an operational definition for that resistance to cell killing by heat which develops during a protracted exposure at temperatures generally in the range of 41.5-42.5 degrees C which is usually observed as a reduction in the slope of the survival curve. While Chinese hamster ovary (CHO) cells are generally more sensitive to high-temperature heat shock than HeLa cells, studies of cells maintained in suspension culture at 41.5 degrees C demonstrated CHO cells to be more resistant to cell killing at this temperature than HeLa cells, due to the expression of chronic thermotolerance in the hamster cell line and the corresponding lack of chronic thermotolerance expression in the HeLa cell line. Experiments were conducted in the two cell lines while heating under identical conditions, in order to detect any cell line-specific changes in heat-induced perturbation of cell cycle progression and the expression of chronic thermotolerance. Our results showed that CHO cells exhibited a G1 block which lasted throughout the course of the 32-h heating period. HeLa cells, however, failed to accumulate in G1, progressing instead into S phase where spontaneous premature chromosome condensation and nuclear fragmentation were observed. This accumulation of cells with condensed chromatin possessing S-phase DNA content exhibited a linear, one-to-one functional relationship with the fraction of dead cells. Previous studies (M.A. Mackey and W.C. Dewey, Int. J. Hyperthermia, 5:405-415, 1989) demonstrated that synchronized S-phase CHO cells heated at 41.5 degrees C and 42 degrees C were unable to express chronic thermotolerance. Therefore, we hypothesize that progression of cells out of G1 phase into S and G2-M phases leads to lethal processes that prevent the expression of chronic thermotolerance in the HeLa cell line. This hypothesis is strengthened by the observed correlation between the accumulation of "mitotic-like" cells and decreased survival, suggesting that the G1 block observed in CHO cells is causally connected with the expression of chronic thermotolerance.