Hyperthermic Killing and Hyperthermic Radiosensitization in Chinese Hamster Ovary Cells: Effects of pH and Thermal Tolerance

Abstract

To quantitatively relate heat killing and heat radiosensitization, asynchronous or G1 Chinese hamster ovary (CHO) cells at pH 7.1 or 6.75 were heated and/or X-irradiated 10 min later. Since no progression of G1 cells into S phase occurred during the heat and radiation treatments, cell cycle artifacts were minimized. However, results obtained for asynchronous and G1 cells were similar. Hyperthermic radiosensitization was expressed as the thermal enhancement factor (TEF), defined as the ratio of the D0 of the radiation survival curve to that of the D0 of the radiation survival curve for heat plus radiation. The TEF increased continuously with increased heat killing at 45.5 degrees C, and for a given amount of heat killing, the amount of heat radiosensitization was the same for both pH's. When cells were heated chronically at 42.4 degrees C at pH 7.4, the TEF increased initially to 2.0-2.5 and then returned to near 1.0 during continued heating as thermal tolerance developed for both heat killing and heat radiosensitization. However, the shoulder (Dq) of the radiation survival curve for heat plus radiation did not manifest thermal tolerance; i.e., it decreased continuously with increased heat killing, independent of temperature, pH, or the development of thermotolerance. These results suggest that heat killing and heat radiosensitization have a target(s) in common (TEF results), along with either a different target(s) or a difference in the manifestation of heat damage (Dq results). For clinical considerations, the interaction between heat and radiation was expressed as (1) the thermal enhancement ratio (TER), which is the dose of X rays alone divided by the dose of X rays combined with heat to obtain an isosurvival, e.g., 10(-4), and (2) the thermal gain factor (TGF), the ratio of the TER at pH 6.75 to the TER at pH 7.4. Since low pH reduced the rate of development of thermal tolerance during heating at low temperatures, low pH enhanced heat killing more at 42-42.5 degrees C than at 45.5 degrees C where thermal tolerance did not develop. Therefore, the increase in the TGF after chronic heating at 42-42.5 degrees C was greater than after acute heating at 45.5 degrees C, due primarily to the increase in heat killing causing an even greater increase in heat radiosensitization. These findings agree with animal experiments suggesting that in the clinic, a therapeutic gain for tumor cells at low pH may be greater for temperatures of 42-42.5 degrees C than of 45.5 degrees C.

Thermal radiosensitization in Chinese hamster (V79) and mouse C3H 10T 1/2 cells. The thermotolerance effect

“…Our data showed a temporal correlation between the onset of thermal tolerance and the decrease in thermal enhancement of radiosensitivity. Likewise, thermotolerance and decreased thermal radiosensitization are also correlated in other studies of (Holahan et al, 1982;Miyakoshi et al, 1979 andHenle et al, 1979). All these results indicate that thermal tolerance in radiosensitization can also occur and these results correlate with the onset of thermotolerance in vitro.…”

“…In a study by Freeman et al (1979) it was found that in CHO cells thermal radiosensitization did not increase with increased heating time at 42.0°C after they have attained a thermotolerant state while the results of Holahan et al (1982) show that there was an actual decrease in thermal radiosensitization as cells attained thermal tolerance. Since these results differed from the results with V79 cells and C3H-1OT 1/2 cells we further investigated this phenomenon at a lower temperature such as 41.0°C.…”

Effects of Hyperthermia on the Linear and Quadratic Parameters of the Radiation Survival Curve of Mammalian Cells: Influence of Thermotolerance

“…An initial rapid rise in TER (a) and TER (/3) can be observed followed by a slower increase . The initial rise was not observed with CHO cells by Holahan et al (1984) . Their experimental conditions were different from ours : they used a 10 min interval between heat treatment and irradiation at 37°C whereas we irradiated during hyperthermia; this also explains the lower TER values observed with CHO cells .…”

“…Only a few papers have been published presenting data on the influence of hyperthermia and thermal tolerance on a (parameter describing the linear component, determining the initial slope of the radiation dose survival curve) and on f3 (parameter describing the quadratic component) in addition to data on D o and N (Hartson-Eaton et al . 1984al . , Holahan et al .…”

Gene transfer into human prostate adenocarcinoma cells with an adenoviral vector: Hyperthermia enhances a double suicide gene expression, cytotoxicity and radiotoxicity

“…18 Thermal stress also enhances radiation -induced cytotoxicity. 19 In this study, we observed that mild hyperthermia ( heating at 418C ), which is clinically achievable, enhanced cytomegalovirus (CMV ) promoterdriven toxin gene expression by 5-to 20 -fold in prostate cancer cells transduced by adenovirus. The findings reported in this paper may still not overcome the low delivery efficiency of therapeutic genes in vivo.…”