Protein Degradation in CHL V79 Cells during and after Exposure to 43°C

Cellular protein degradation during and following hyperthermia should be altered due to increased enzymatic activity at elevated temperatures, inhibition of protein synthesis, and denaturation of proteins. We have previously demonstrated by differential scanning calorimetry that approximately 1-2% of total CHL V79 cellular protein denatures during a 10- to 15-min exposure to 43°C (J. E. Lepock et al., J. Cell. Physiol. 137, 14-24 (1988)). Proteolysis was measured during and after exposure to 43°C. The decay curves of the degradation of $[{}^{3}{\rm H}]\text{Leu-labeled}$ proteins are fit well by a double exponential; however, each component is the sum of the decay curves of a large number of proteins, probably with a distribution of rates of degradation. At 37°C a fast-decaying component $(T_{1/2}\simeq 1.3\ {\rm h})$, representing short-term proteins, and a slow-decaying component $(T_{1/2}\simeq 50\ {\rm h})$, representing long-term proteins, are observed. At 43°C the rate of degradation of the fast-decaying component is stimulated three- to fivefold (to $T_{1/2}=0.27-0.45\ {\rm h}$). After return to 37°C, the rate of degradation of the slow-decaying component is depressed twofold (to $T_{1/2}=109-141\ {\rm h}$). The period of depression is dose dependent (i.e., time at 43°C) and recovers at approximately the same time as resumption of protein synthesis and growth. Overall stimulation of degradation lasts for approximately 15 min at 43°C and, coupled with an inhibition of synthesis, leads to the loss of at least a small percentage of total cellular protein. It is likely that the initial stimulated degradation is in part due to increased substrate in the form of denatured protein, further supporting the denaturation of proteins during hyperthermia.