Induction of Thermotolerance and Evidence for a Well-Defined, Thermotropic Cooperative Process

Abstract

Heat-induced thermal resistance is not only an interesting biological phenomenon, but may also present an important consideration in the application of hyperthermia in cancer therapy. Characterization of the temperature dependence and kinetics of the induction of thermotolerance serves two purposes. First, such data are essential to the rational design of clinical heat treatment protocols, and second, knowledge of the kinetics and temperature dependence may yield some insight into the mechanism of thermotolerance and thermal inactivation. Split-dose experiments were performed using plateau-phase Chinese hamster HA-1 cells. The effects of varying the temperature (41-470C) and the duration of the first heat treatment on the subsequent expression of thermotolerance were studied. Our results indicate that cells initially treated at 41 and 420C are essentially at their most resistant state at the end of the first treatment, while an initial exposure to 430C or higher requires approximately 8 hr of incubation at 370C before maximal thermotolerance is expressed. Based on our three-component model of thermotolerance (trigger-development-decay), we found that neither the rate of development nor the rate of decay was significantly affected by the duration of the first treatment. By plotting the survival at maximal thermotolerance as a function of the duration of the initial heat treatment (at temperatures of 40-470C), we were able to display the kinetics of the trigger. From these data we constructed an Arrhenius plot which yielded two linear, parallel segments with an unusually sharp discontinuity near 43?C. Over both temperature ranges, 43-47 and 40-42.50C, the activation energy is approximately 120 kcal/mole, consistent with the involvement of protein structure alterations in the heat-actuated event. Furthermore, the discontinuity in the Arrhenius plot implies the existence of a sharp thermotropic cooperative process occurring at approximately 430C.