Hepatoma cells adapted to proliferate under normally lethal hyperthermic stress conditions show rapid decay of thermoresistance and heat shock protein synthesis when returned to 37°C

H35 hepatoma cultures were adapted to sustained growth at 41·3°C. In these variant cells the 'basic' levels of various heat shock proteins (hsps), especially those of hsp60, 70 and 100, are significantly raised. These cells exhibit a thermoresistance comparable with the induced thermotolerance in normal hepatoma cells heat shocked at 42·5°C for 30 min. However, this resistance of variant cells shows a rapid, exponential decay with a half-time of 2·2 h when the temperature is lowered to 37°C, with a concomitant decrease of the synthesis of hsp60 and 70. Heat shock experiments with variant cells grown at 41·3°C lead to increased thermoresistance and synthesis of hsps when further incubation was performed at the original temperature but not at 37·°C. In the latter case, only a 3-h delay in the onset of decay of thermoresistance is observed. However, when the variant cells were incubated at 37°C prior to heat stress normal induction of thermoresistance and hsp synthesis return inversely proportional to the progression of thermoresistance decay. Thermoresistant cells thus seem to be valuable tools in the study of the down-regulation of thermoresistance as well as of hsp synthesis.