Mechanism of drug-induced heat resistance: the role of protein degradation?

To investigate the possibility that heat-induced protein degradation may play a role in heat killing of mammalian cells, we have compared cellular survival and protein degradation rates for cells treated with cycloheximide, puromycin, or histidinol. These three compounds all inhibit protein synthesis and protect against the lethal effects of heat shock. When cells were treated with histidinol for 2 h before heating, as well as during heating at 43 degrees C for 3 h, they became resistant to heat killing. Histidinol treatment (5 mM) induced a 10,000-fold increase in surviving fraction from 10(-5) to 10(-1), and the protective effect was similar to that of 0.1 mM cycloheximide or 0.2 mM puromycin. Despite the similarity in heat protection for the three compounds, the protein degradation rate of 1.8%/h at 37 degrees C was increased by 34% by histidinol and decreased 20% by cycloheximide or puromycin. At 43 degrees C none of these compounds had a significant effect on the protein degradation rate. Therefore the intracellular degradation of relatively long-lived proteins does not appear to play a significant role in either heat killing or the phenomenon of heat protection. Instead, since maximum protection from heat killing was observed for all three compounds when protein synthesis was inhibited by 90-95%, heat protection probably results from an event(s) that is caused by inhibition of protein synthesis.