Hyperthermia-induced radiosensitization in CHO wild-type, NHEJ repair mutant and HR repair mutant following proton and carbon-ion exposure

The DNA repair mechanisms involved in hyperthermia-induced radiosensitization with proton and carbon ion radiation exposure were investigated in the present study. In a previous study, Chinese hamster ovary (CHO) cells were exposed to low linear energy transfer (LET) photon radiation. These cells can be sensitized by hyperthermia as a result of inhibition of homologous recombination (HR) repair. The present study used wild-type, non-homologous end joining (NHEJ) and HR repair-deficient CHO cells to define the contributions of each repair pathway to cellular lethality following hyperthermia-induced hadron radiation sensitization. The cells were exposed to ionizing radiation, followed by hyperthermia treatment (42.5°C for 1 h). Hyperthermia-induced radiosensitization was determined by the colony formation assay and thermal enhancement ratio. HR repair-deficient cells exhibited no hyper-sensitization to X-rays, protons, or low and high LET carbon ions when combined with hyperthermia. Wild-type and NHEJ repair-deficient cells exhibited significant hyperthermia-induced sensitization to low LET photon and hadron radiation. Hyperthermia-induced sensitization to high LET carbon-ion radiation was less than at low LET radiation. Relative biological effectiveness (RBE) between radiation alone and radiation combined with hyperthermia cell groups was not significantly different in any of the cell lines, with the exception of wild-type cells exposed to high LET radiation, which exhibited a lower RBE in the combined group. The present study investigated additional cell lines to confirm the lower RBE observed in DNA repair-deficient cell lines. These findings suggested that hyperthermia-induced hyper-sensitization to hadron radiation is also dependent on inhibition of HR repair, as was observed with photon radiation in a previous study.

Hyperthermia, cisplatin and radiation trimodality treatment: a promising cancer treatment? A review from preclinical studies to clinical application

This review discusses available clinical and experimental data and the underlying mechanisms involved in trimodality treatment consisting of hyperthermia, cisplatin and radiotherapy. The results of phase I/II clinical trials show that trimodality treatment is effective and feasible in various cancer types and sites with tolerable toxicity. Based on these results, phase III trials have been launched to investigate whether significant differences in treatment outcome exist between trimodality and standard treatment. In view of the clinical interest, it is surprising to find so few preclinical studies on trimodality treatment. Although little information is available on the doses of the modalities and the treatment sequence resulting in the largest degree of synergistic interaction, the results from in vivo and in vitro preclinical studies support the use of trimodality treatment for cancer patients. Animal studies show an improvement in treatment outcome after trimodality treatment compared with mono- and bimodality treatment. Studies in different human tumour cell lines show that a synergistic interaction can be obtained between hyperthermia, cisplatin and radiation and that this interaction is more likely to occur in cell lines which are more sensitive to cisplatin.

Mechanism of radiosensitization by hyperthermia (> or = 43 degrees C) as derived from studies with DNA repair defective mutant cell lines

All biochemical and cytogenetic data on radiosensitization by heat treatment at and above 43 degrees C indicate that inhibition of DNA repair plays a central role. There are several DNA repair pathways involved in restoration of damage after ionising irradiation and the kinetics of all of them are affected by heat shock. This, however, does not imply that the inhibition of each of these pathways is relevant to the effect of heat on cellular radiosensitivity. The current review evaluates the available data on heat radiosensitization in mutant or knockout cell lines defective in various DNA repair proteins and/or pathways. The data show that thermal inhibition of the non-homologous end-joining pathway (NHEJ) plays no role in heat radiosensitization. Furthermore, limited data suggest that the homologous recombination pathway may also not be a major heat target. By deduction, it is suggested that inhibition of base damage repair (BER) could be the crucial step in radiosensitization by heat. While a lack of mutant cell lines and redundancy of the BER pathway have hampered efforts toward a conclusive study, biochemical and correlative evidence support this hypothesis.

Survival and recovery of yeast cells after simultaneous treatment of UV light radiation and heat

Cell survival, synergistic interaction, liquid-holding recovery (LHR) kinetics and inactivation forms after the simultaneous treatment with UV light (254 nm) and various high temperatures were studied in diploid yeast cells Saccharomyces cerevisiae. The synergistic interaction was observed within a certain temperature range in which there was a temperature that maximizes the synergistic effect. The LHR study revealed that both the extent and the rate of recovery greatly decreased with the increase in exposure temperature. A quantitative approach describing the LHR process as a decrease in the effective radiation dose was used to estimate the probability of recovery per unit time and the irreversible component of damage. Using the experimental data obtained and the mathematical model described, it was shown that the irreversible component, i.e. the fraction of cells incapable of recovery, increased with the exposure temperature, whereas the recovery constant, i.e. the probability of recovery per unit time, was independent of the exposure temperature. The increase in the irreversible component was accompanied by an increase in cell death without postirradiation division. It is concluded based on this that the synergistic interaction of UV light radiation and hyperthermia in yeast cells is not related to the impairment of the recovery process itself and that it may be attributed to an increased yield of the irreversible damage.

Evaluation of Recombination Repair Pathways in Thermal Radiosensitization

Thermal radiosensitization has been shown to cause inhibition of repair of sublethal and potentially lethal damage and DNA DSBs. In this study we assessed thermal radiosensitization in mutants deficient in homologous recombinational (HR) repair and nonhomologous end joining repair (NHEJ). Using cells of the mouse wild-type embryo fibroblast cell line MEF and its Ku80(-/-) derivative that is deficient in NHEJ, we showed that thermal radiosensitization is the same in both cell lines. Further studies with cells of the wild-type CHO-AA8 cell line and its derivative IRS(ISF), which is deficient in HR, also showed comparable thermal radiosensitization in both cell lines. Further experiments using cells of chicken DT40 cell lines also showed comparable thermal radiosensitization between the wild-type HR mutant Rad54, the NHEJ mutant Ku70, and the double mutant Rad 54-Ku70. These results indicate that the HR and NHEJ pathways may not be targets for thermal radiosensitization.

Fluence rate as a determinant of synergistic interaction under simultaneous action of UV light and mild heat in Saccharomyces cerevisiae

In experiments with wild-type diploid yeast cells of Saccharomyces cerevisiae, the synergistic lethal action of a simultaneous application of ultraviolet (UV) light (wavelength, 254 nm) and mild heat (45-57.5 degrees C) was studied. It was shown that, at any fixed UV light intensity, the synergistic effect occurred within the given temperature interval. The optimal temperature to achieve the greatest synergistic effect may be shown for every fluence rate examined. The correlation between the optimal temperature that maximized the synergy and UV light intensity was estimated: this temperature shifted towards higher temperature values with an increasing fluence rate. A possible interpretation of this effect is based on the supposition that the mechanism of the synergistic effect is related to additional lethal damage produced by the interaction of sublesions induced by each agent. These sublesions are supposed to be non-lethal when each agent is applied separately.

Water bath hyperthermia is a simple therapy for psoriasis and also stimulates skin tanning in response to sunlight

An eight week trial, involving superficial hyperthermia delivered biweekly via simple water bath immersion, was tested for its ability to clear mild to moderate psoriatic lesions. Seven patients were treated and three cases rapidly improved. In the remaining patients, the treatment frequency was increased to alternate days; two cases improved significantly, one patient showed a partial response, and the fourth had no visible change (this was the only patient taking concurrent drug therapy--etretinate). In addition to resolving psoriatic lesions, water bath hyperthermia also reduced edema (swelling) and relieved pruritus (itching) in all patients, both during the treatment period and for up to several months after lesions had returned. Lesion reappearance occurred within one to three months after the last heat treatment. We retreated one patient and produced a second complete remission. These results indicate that simple repetitive water bath hyperthermia alone is effective in the treatment of psoriatic lesions in heatable locations. An unexpected side effect was enhanced melanin content (tanning) in all areas where hyperthermia treated skin was exposed to sunlight.

Hyperthermia-induced modulation of killing and mutation by UV and N-methyl-N'-nitro-N-nitrosoguanidine in V79 cells

Hyperthermic exposures of V79 cells did not affect the killing by UV light, whereas it enhanced MNNG-induced killing. Such hyperthermic exposure increased the mutation induction (resistance to 6-thioguanine) by both UV and MNNG. The timing of heat exposure, before or after the treatments, had no effect on the result in cases of cytotoxicity and mutagenesis.

Effects of hyperthermia on mouse L cells irradiated with fractionated X-rays

The responses to X-rays and radiosensitizing effects of post-treatments with heat were examined in the cells exposed to different doses of multifractionations. When cells were irradiated with daily doses of 2 Gy the radiation responses changed during multifractionated treatments, giving rise to two components in survival curves. The changes may be due to selective killing of radiosensitive cells or a cell progression through radioresistant phases during multifractionation irradiation as shown in analysis of the cell cycle. However, the survival curves became almost exponential when fractionation was performed with daily doses of 3-4 Gy. Radiosensitization by heat at 43 degrees C for 15 min was detectable in the cells after exposures to fractionated doses of more than 7 Gy with daily doses of 2 Gy, but not after exposures to daily doses of 3-4 Gy. Radiosensitization by heat at 43 degrees C for 30 min was manifest as disappearance of shoulder and increase of the slope in survival curves after irradiation with daily doses of 2 Gy. It was observed as an increase of slope in survival curves when cells were irradiated with daily doses of 3 or 4 Gy. Such differences in the radiosensitization may be due to a difference in the susceptibility to heat among the cells which had been exposed to the different doses and regimens of fractionated irradiation.