Thermal adaptation in CHO cells at 40 degrees C: the influence of growth conditions and the role of heat shock proteins.

The kinetics of thermal adaptation at the nonlethal temperature of 40 degrees C was studied in CHO (Chinese hamster ovary) cells in vitro. Thermal resistance, demonstrated as an increase in mean 45 degrees C killing time or as an increase in the shoulder of the 45 degrees C survival curve, was fully developed by 2 h. Control cells in early logarithmic phase were more heat sensitive than those in stationary phase. Corresponding 45 degrees C killing time frequency distributions were unimodal with an increase in mean killing time from early logarithmic to stationary phase. Cells which were thermally adapted at 40 degrees C for 6 h had biphasic 45 degrees C killing time frequency distributions, and as cells progressed from early logarithmic to stationary phase the heat-sensitive subpopulation progressively declined. Exposure to 40 degrees C produced a 30% increase in total protein synthesis. Proteins with molecular weights 72, 89, and 109 kDa which correspond to those induced by lethal heat shock were synthesized at 40 degrees C, but there was no close temporal correlation between the development of heat resistance at 40 degrees C and synthesis of the heat shock proteins. Cycloheximide (100 micrograms/ml) reduced the mean 45 degrees C killing time but did not totally prevent the development of heat resistance at 40 degrees C.