The effect of step-down heating on mouse small intestinal mucosa

Effects of step-down and step-up heating on the development of thermotolerance in a C3H mammary carcinoma in vivo

The effects of step-down (SDH) and step-up heating (SUH) on the development of thermotolerance were investigated in a C3H mammary carcinoma in vivo. The endpoint was tumour growth time, i.e. the time for a tumour to reach a volume five times that of the first treatment day. SDH consisted of 44.5 degrees C/5 min followed immediately by 41.0 degrees C/120 min. SUH consisted of the same heat treatments but in reverse sequence. Thermotolerance was detected by subsequent heating at 43.5 degrees C at variable intervals following the primary SDH or SUH. The degree of thermotolerance was quantified by the thermotolerance ratio (TTR) calculated as a ratio between the slope of the dose-response curve for tumours heated at 43.5 degrees C and tumours preheated with either SDH or SUH followed by 43.5 degrees C. Both SDH and SUH induced thermotolerance. However, the maximal degree of thermotolerance and the time interval to reach maximum thermotolerance were different. For SUH maximal thermotolerance was observed at 8 h with a TTR of 3.6. For SUH, thermotolerance peaked at 24-28 h with a TTR of 7.3. In both cases thermotolerance had decayed with a 120 h interval. The SDH priming induced about 2.5 times more heat damage than SUH. The results are therefore in agreement with previous data obtained in the same tumour model by single heating showing that both the degree and the time to reach maximal thermotolerance increases with pretreatment heat damage. In addition, the results indicate that thermotolerance and thermosensitization are independent phenomena.

Step-down heating of human melanoma xenografts: effects of the tumour microenvironment

Thermosensitisation by step-down heating (SDH) has previously been demonstrated in experimental rodent tumours. The purpose of the study reported here was to investigate whether the SDH effect in tumours in part may be attributed to heat-induced alterations in the capillary network and/or the microenvironment. Two human melanoma xenograft lines differing substantially in vascular parameters were selected for the study. A thermostatically regulated water bath was used for heat treatment. The conditioning treatment (44.5 degrees C or 45.5 degrees C for 15 min) was given in vivo, whereas the test treatment (42.0 degrees C for 45, 90, 135 or 180 min) was given either in vitro or in vivo. Treatment response was measured in vitro using a cell clonogenicity assay. Fraction of occluded vessels following heat treatment was assessed by examination of histological sections from tumours whose vascular network was filled with a contrast agent. Tumour bioenergetic status and tumour pH were measured by 31P magnetic resonance spectroscopy. The conditioning heat treatments caused significant vessel occlusion, decreased tumour bioenergetic status and decreased tumour pH in both tumour lines. The SDH effect measured when the test treatment was given in vivo was significantly increased relative to that measured when the test treatment was given in vitro. The magnitude of the increase showed a close relationship to fraction of occluded vessels, tumour bioenergetic status and tumour pH measured 90 min after treatment with 44.5 degrees C or 45.5 degrees C for 15 min. The increased SDH effect in vivo was probably attributable to tumour cells that were heat sensitive owing to the induction of low nutritional status and pH during the conditioning treatment. Consequently, the SDH effect in some tumours may in part be due to heat-induced alterations in the microenvironment. This suggests that SDH may be exploited clinically to achieve increased cell inactivation in tumours relative to the surrounding normal tissues.

Mild step-down heating causes increased levels of HSP68 and of HSP84 mRNA and enhances thermotolerance

Mammalian cells exhibit increased sensitivity to hyperthermic temperatures of 38-42 degrees C after an acute high-temperature heat shock; this phenomenon is known as thermo-sensitization or the step-down heating effect. In order to determine whether the increase of heat shock mRNA after heat stress can be thermosensitized, we studied the induction of the mRNA of HSP68 and of HSP84 after application of step-down heating (SDH) in Reuber H35 rat hepatoma cells. SDH consisted of a pretreatment of 30 min at 41.5, 42.5 or 43.5 degrees C, followed by a continuous incubation at a lower hyperthermic temperature (40 or 41 degrees C). After mild pretreatment (30 min at 41.5 degrees C) the mRNA level of HSP68 was increased by subsequent incubation at 40 degrees C, although incubation at 40 degrees C alone had no effect. This increase was even more pronounced at 41 degrees C. An increase in the level of HSP84 mRNA was also observed after mild pretreatment (41.5 degrees C/30 min) followed by 41 degrees C post-incubation. Interestingly, an enhanced occurrence of thermotolerance was also observed upon application of mild step-down heating (42 degrees C/30 min-40 degrees C-43.5 degrees C/60 min). In contrast, cell cultures treated for 30 min at 43.5 degrees C (a temperature which induces an increase in HSP mRNA levels) showed an inhibited or delayed synthesis of HSP mRNA when post-treated at 40 or 41 degrees C. Under these conditions the development of thermotolerance did not take place either. With respect to the effect of step-down heating on HSP mRNA levels as well as on thermotolerance development, our data imply that a distinction should be made between 'mild' and 'severe' pretreatment temperature of the step-down heating protocol.

Winner of the Lund Science Award 1992. Thermosensitization induced by step-down heating. A review on heat-induced sensitization to hyperthermia alone or hyperthermia combined with radiation

A few minute's exposure to a high temperature (sensitizing treatment, ST) may substantially increase the cytotoxic and the radiosensitizing effect of a subsequent heating at a lower temperature (test treatment, TT). This phenomenon, which is known as step-down heating (SDH) or thermosensitization, has been observed both in cultured cells in vitro and in tumours and normal tissues in vivo. The effect of SDH increases with a lowering of TT temperature, but it is rapidly lost at temperatures very close to 37 degrees C. SDH-induced thermosensitization decays within a few hours, when an interval is inserted between ST and TT. In vitro results suggest an exponential decay of the SDH effect with half times ranging from 1.5- to 3.1 h. The effect of SDH increases with increasing ST time or temperature. For single heating, the Arrhenius plot is biphasic with activation energies of 500-800 and 1200-1700 kJ/mol above and below a break point temperature in the region 42.5-43.0 degrees C, respectively. For SDH, the Arrhenius plot gradually becomes monophasic with increasing severity of ST and it approaches asymptotically to an activation energy of about 400 kJ/mol. The reduction of the activation energy depends on cell survival after the priming ST and not on the specific ST heating time or temperature. SDH strongly enhances hyperthermic radiosensitization with a 5-6-fold reduction of the radiation dose required to achieve tumour control. The thermosensitizing and the radiosensitizing effects of SDH have several features in common. Both effects become more prominent when the TT temperature is decreased and when the ST heating time or temperature increases. In addition, the decay kinetics for both effects are comparable. For heat alone, the effect of SDH in tumour and normal tissue seems to be quantitatively similar. However, the therapeutic ratio may be increased by combining SDH with radiation. Biologically, the critical subcellular targets involved in the SDH effect have not been revealed. However, the ability of SDH to inhibit the clearance of heat-induced aggregation of proteins in the nucleus is interesting. Blockage of the nuclear function by proteins is a central theory in the present molecular biological models for both cell kill by heat and heat radiosensitization. Clinically, SDH may be an advantage since even a short exposure to high temperature increases the effect of an otherwise inadequate heat treatment. The disadvantages are that SDH complicates thermal dose calculations, and may cause unacceptable damage to normal tissue.

Sensitization to hyperthermia induced in a normal tissue by step-down heating

The effect of step-down heating was investigated in the skin of the CDF1 mouse foot. Step-down heating was induced with a 44.7 degrees C/10 min pretreatment followed by a test treatment at a lower temperature for variable time. Step-up heating, that is, a test treatment followed by a 44.7 degrees C/10 min treatment, and single heating were used as controls. The normal tissue reaction was scored at five levels of damage (from slight redness and oedema to loss of a toe or greater reaction), and the heating time to induce each level in 50% of the animals, RD50, was used as the endpoint. The effect of step-down heating was quantified by the step-down ratio, calculated as the ratio of test heating times to obtain the endpoint. A significant reduction of the RD50 was seen at all score levels when the 44.7 degrees C/10 min was given in a step-down heating schedule, and the effect increased with decreasing test treatment temperature. In contrast, the heat sensitivity was only marginally influenced by step-up heating. An analysis of the time-temperature relationship demonstrated a log-linear relationship between temperature and RD50 for single heating in the range 42.2-44.7 degrees C and for step-down heating in the range 41.7-44.7 degrees C. The curve for step-down heating showed a lesser slope indicating a decrease of the activation energy. The kinetics of the SDH effect were investigated by inserting an interval between a primary 44.7 degrees C/10 min treatment and a test treatment performed at 42.2 degrees C. The effect of step-down heating was maximal with no interval between the priming treatment and the test treatment. As the interval was increased to 1.5 hr the step-down sensitization disappeared, and with even longer intervals thermotolerance developed. From a clinical point of view, the present data indicate that step-down heating may increase the extent of both reversible and irreversible heat damage in the normal tissue.

A comparison between the effect of step-down heating in a tumour and a normal tissue in vivo

A comparison between the effect of step-down heating (SDH) obtained in a C3H mammary carcinoma grown in the feet of CDF1 mice and the skin of normal CDF1 feet is presented. Water-bath heating was used, and SDH was obtained by giving a 44.7 degrees C/10 min treatment followed by heating at 42.2 degrees C for variable times. Single heating at 42.2 degrees C and step-up heating (SUH), i.e. 42.2 degrees C followed by 44.7 degrees C/10 min, were used as controls. The endpoint was the heating time at 42.2 degrees C to obtain either a definite tumour growth time (TGT50) or a specific skin score level (RD50) in 50% of the animals. The effect of SDH and SUH was quantified by the step-down ratio (SDR), calculated as the ratio of the heating times at 42.2 degrees C to obtain the specific endpoint. In both assays the effect of SDH was seen as a significant left shift of the SDH dose-response curve compared to the curve for single heating and SUH. For the comparison of the tumour and the normal tissue response, damage levels with comparable heating times for single heating were used. The therapeutic effect was then investigated by calculating the therapeutic gain factor (TGF), where TGF = SDR(tumour)/SDR(normal tissue). Neither SUH nor SDH gave a TGF significantly different from 1. The results suggest that SDH may be used clinically to shorten the heating time without decreasing the therapeutic effect.

Step-down heating in a C3H mammary carcinoma in vivo: effects of varying the time and temperature of the sensitizing treatment

The effect of step-down heating (SDH), consisting of an initial sensitizing treatment (ST) performed at either 44.5 degrees C or 43.5 degrees C followed by a lower temperature test treatment (TT), was investigated in a C3H mammary carcinoma in vivo. A linear relationship between heating time and tumour growth delay was observed for all temperature combinations applied. At a given TT temperature, SDH increased the slope of the dose-response curve compared to the curve for tumours, single-heated without an initial ST. The slope of the SDH curves increased asymptotically towards a plateau value as the ST time at 44.5 degrees C was increased. The time-temperature relationship for single heating was described by a biphasic Arrhenius curve with activation energies of 1361 +/- 34 and 666 +/- 54 kJ/mol below and above an inflection point at 42.5 degrees C, respectively. For SDH, the Arrhenius curve gradually became straight with increasing ST time, and the activation energy saturated at a value of 425 +/- 25 kJ/mol. The reduction of the activation energy at an ST temperature of 43.5 degrees C was due rather to the extent of ST heat damage than to the ST time or temperature used. These results may be relevant for calculations of thermal doses, since even a short temperature peak (e.g. 44.5 degrees C/5 min) significantly changed the time-temperature relationship.

Step-down heating of CHO cells at 37.5-39 degrees C

The thermosensitizing effect of step-down heating was studied using Chinese hamster ovary (CHO) cells. Exponentially growing cells were given priming heat treatments at 43 degrees C for 45 or 90 min, immediately followed by a second exposure to a temperature ranging from 37.5 to 39 degrees C. The measured rates of cell killing, 1/D0, increased exponentially with temperature; the slopes correspond to Arrhenius activation energies of Ea = 1200 +/- 150 kJ mol-1 and Ea = 1275 +/- 125 kJ mol-1 for cells preheated at 43 degrees C for 45 or 90 min, respectively. For the temperature range 39-43 degrees C an activation energy of Ea = 561 +/- 24 kJ mol-1 was obtained for step-down heated cells (43 degrees C, 45 min followed by T = 39-43 degrees C). These results indicate that there is a 'second inflection point' at 39 degrees C on the Arrhenius curve for step-down heating of CHO cells. Data evaluation using a mathematical model published previously (H. Jung, Radiation Research, 106, 56-72, 1986) showed that the rate constant c for the conversion of nonlethal lesions into lethal events increased with an activation energy of Ea = 1520 +/- 140 kJ mol-1 in the temperature range from 37.5-39 degrees C. For 39-45 degrees C the activation energy for c was Ea = 360 +/- 26 kJ mol-1, indicating that the temperature dependence of c shows a break at 39 degrees C similar to that observed on the 1/D0 Arrhenius plot.

Thermosensitization by step-down heating in mouse testis

Step-down heating (SDH) was investigated in mouse testis by giving an initial treatment of 3 min at 43.0 degrees C followed immediately by a treatment in the temperature range 38.0-42.0 degrees C. The dose-response curves for testis weight loss as a function of duration of hyperthermia were compared with those obtained using single-temperature treatments. In all cases the curves were linear, allowing the use of Arrhenius analysis. For single-temperature treatments the Arrhenius relationship showed an inflection at approximately 41 degrees C with a small, but significant, increase in activation energy for hyperthermal temperatures below the transition. SDH increased the thermal sensitivity in this lower range, by approximately 1 degree C, but the activation energy was not significantly altered. The results support the view that in vivo thermosensitization by SDH is not due solely to inhibition of development of thermotolerance.