Kurt G. Hofer

Ionizing-radiation resistance in the desiccation-tolerant cyanobacterium Chroococcidiopsis.

ABSTRACT The effect of X-ray irradiation on cell survival, induction, and repair of DNA damage was studied by using 10Chroococcidiopsis strains isolated from desert and hypersaline environments. After exposure to 2.5 kGy, the percentages of survival for the strains ranged from 80 to 35%. In the four most resistant strains, the levels of survival were reduced by 1 or 2 orders of magnitude after irradiation with 5 kGy; viable cells were recovered after exposure to 15 kGy but not after exposure to 20 kGy. The severe DNA damage evident after exposure to 2.5 kGy was repaired within 3 h, and the severe DNA damage evident after exposure to 5 kGy was repaired within 24 h. The increase in trichloroacetic acid-precipitable radioactivity in the culture supernatant after irradiation with 2.5 kGy might have been due to cell lysis and/or an excision process involved in DNA repair. The radiation resistance ofChroococcidiopsis strains may reflect the ability of these cyanobacteria to survive prolonged desiccation through efficient repair of the DNA damage that accumulates during dehydration.

Radiotoxicity of Intracellular 67Ga, 125I and 3H

SummaryL1210 leukaemia cells were labelled with various doses of 67Ga-citrate, 3H-thymidine, or 125I-iododeoxyuridine to evaluate the cytocidal effects of the intracellular decay of the three radionuclides. Based on radioisotope incorporation data, cellular dimensions, and intracellular radioisotope distributions (3H and 125I intranuclear, 67Ga cytoplasmic) the rates of deposition of cellular, nuclear, and cytoplasmic energy were calculated.In terms of energy absorption/cell, 67Ga (LD50: 2250 keV/hr; 69 rad/hr) was much less toxic than either 3H (LD50: 325 keV/hr; 10 rad/hr) or 125I (LD50: 50 keV/hr; 1·5 rad/hr). In terms of energy absorption/nucleus, 67Ga and 3H produced almost identical effects (LD50: 230 versus 255 keV/hr; 22·2 versus 24·6 rad/hr), but 125I remained much more toxic (LD50: 40 keV/hr; 3·9 rad/hr).These findings indicate that, although decay by electron capture in the cell nucleus (125I) is highly destructive, the same type of decay occurring in the cytoplasm (67Ga) is ineffective in kill...

Biophysical aspects of Auger processes.

In this review I begin with a brief history of Auger processes, and then present a more detailed review of recent literature reports on physical, molecular, and cellular aspects of Auger emitters, and potential therapeutic applications.

Antihistamines block radiation-induced taste aversions.

When rats are treated with an antihistamine prior to being given sublethal doses of ionizing radiation, the formation of a conditioned saccharin aversion is completely inhibited. A profound aversion could be conditioned with histamine diphosphate as the aversive stimulus. The increase in histamine production after radiation exposure represents the physiological basis of radiation-induced taste aversions.

Low-LET and high-LET radiation action of 125I decays in DNA: effect of cysteamine on micronucleus formation and cell killing.

Chinese hamster ovary cells were pulse-labeled with 125I-iododeoxyuridine during early S phase, and cell samples were harvested 30 min or 5 h after labeling. The samples were frozen (with or without 25 mM cysteamine) and stored at -196 degrees C for accumulation of 125I decays. X-ray control experiments were performed at 37 degrees C and -196 degrees C. Aliquots of cells were plated for evaluating micronucleus formation and cell survival. The results demonstrated a striking shift in micronucleus formation and cell death with time after labeling. Cells frozen 30 min after labeling exhibited effects typical of low-LET radiation, but cells frozen 5 h after labeling showed a response characteristic of high-LET radiation. Cysteamine provided protection against the effects of 125I during the initial phase of effects characteristic of low-LET radiation, but no protection was seen during the phase characteristic of high-LET radiation. When cell survival was evaluated as a function of micronucleus frequency rather than dose in decays/cell, the survival curves for all treatment groups became superimposed. Previous work using the same experimental system had failed to show a direct link between 125I-induced DNA double-strand breaks and cell death. These findings are consistent with the hypothesis that DNA damage may not be the sole mechanism for cell killing and that damage to higher-order structures in the cell nucleus may contribute to (or modify) radiation-induced cell death.

The paradoxical nature of DNA damage and cell death induced by 125I decay

Chinese hamster ovary cells were synchronized at the G1/S-phase boundary of the cell cycle and pulse-labeled for 10 min with 125I-iododeoxyuridine 30 min after entering the S phase. Cell samples were harvested for freezing and 125I-decay accumulation at intervals ranging from 15 to 480 min after termination of labeling. The survival data showed a marked shift from cell killing characteristic of low-LET radiation to that more characteristic of killing by high-LET radiation with increasing intervals between DNA pulse-labeling and decay accumulation. Cells harvested and frozen within 1 h after pulse-labeling yielded a low-LET radiation survival response with a pronounced shoulder and a large D0 of up to 0.9 Gy. With longer chase periods the shoulder and the D0 decreased progressively, and cells harvested 5 h after pulse-labeling or later exhibited a high-LET survival response (D0: 0.13 Gy). Two interpretations for these findings are discussed. (1) If DNA is the sole target for radiation death, the results indicate that DNA maturation increases radiation damage to DNA or reduces damage repair. (2) If radiation cell death involves damage to higher-order structures in the cell nucleus, the findings suggest that newly replicated DNA is not attached to these structures during the initial low-LET period, but 125I starts to induce high-LET radiation effects as labeled DNA segments become associated with the target structure(s). On balance, or data favor the latter interpretation.

Effect of hyperthermia on the radiosensitivity of normal and malignant cells in mice.

The effect of systemic hyperthermia on the in vivo radiation response of normal and malignant mouse cells was evaluated. X‐irradiation of L1210 cells and Ehrlich ascites cells at body temperatures above 41°C resulted in strongly enhanced tumor cell death. The magnitude of this thermal effect increased with increasing temperatures. Hypoxic tumor cells were particularly sensitive to combined heat‐radiation treatment. L1210 leukemia cells did not become resistant to the sensitizing effects of hyperthermia even after repeated heat exposures over several transplant generations. The sensitizing action of hyperthermia varied with different heating strategies. Heating before or during irradiation did not materially alter the radiation response of tumor cells. Maximal potentiation of radiation damage was achieved only when the tumorous mice were subjected to at least 20 minutes heat incubation after irradiation. LD50(30) studies on ICR mice revealed that moderate hyperthermia (41.5°C) does not alter the radiation response of normal body tissues. These findings indicate that it is possible to devise hyperthermic treatment regimens that drastically enhance radiation‐induced tumor cell death in vivo without reducing the radioresistance of normal body tissues.

Role of Mitochondrial DNA in Cell Death Induced by 125I Decay

The role of mitochondrial DNA in radiation-induced cell death was determined by selective [125I]iododeoxyuridine (125IUdR) incorporation into exclusively nuclear sites compared to labelling in both nuclear and mitochondrial DNA of Chinese hamster cells. Such selectivity was achieved by using berenil (25 micrograms/ml for 24 h), a drug which inhibits mitochondrial DNA synthesis without affecting incorporation of 125IUdR into nuclear DNA but does not result in reduced clonogenicity or cell cycle perturbations or alteration in the X-ray response of cells. There was no difference in cell killing between cells with nuclear labelling alone compared with nuclear plus mitochondrial labelling. The absence of decays in mitochondrial DNA does not affect the ability of 125I to induce lethal cell damage. The two treatment groups have superimposable curves with a D0 of 96 decays/cell. These findings indicate that mitochondrial DNA is not the most sensitive target for radiation-induced cell death from 125I decay.

Inhomogeneity of the nucleus to 125IUdR cytotoxicity

Synchronized suspension cultures of Chinese hamster ovary (CHO) cells were used to determine the lethal effects produced by the decay of 125I incorporated into different subfractions of the nuclear genome. Such a shift in nuclear incorporation pattern was achieved by using the drug aphidicolin, which inhibits 95% of all nuclear DNA synthesis, is nontoxic to cells in a colony-forming assay, and does not modify the radiation response of CHO cells to X irradiation. In addition to shifting incorporation of 125I to only 5% of the nuclear genome, both nuclease digestions to characterize the molecular location of 125I and electron microscope autoradiography show an inhomogeneous distribution of sites of 125I incorporation in the presence of 5 micrograms/ml aphidicolin. These data in combination with survival curves of CHO cells labeled with 125I-iododeoxyuridine (125IUdR) either with or without aphidicolin showed a dramatic change in the survival response (DO: 30 decays/cell and 96 decays/cell, respectively). It is concluded, therefore, that the nucleus is not a homogeneous target for radiation-induced cell death because when subfractions of the nuclear genome are labeled, radically different levels in cell survival are obtained.

Evidence for separate modes of action in thermal radiosensitization and direct thermal cell death

It is not known whether heat potentiation of radiation damage and direct heat death are mediated by the same or by different heat lesions within the cell. In this study, three types of experiments were performed on BP‐8 murine sarcoma cells to provide evidence against a common mode of action: (1)Evaluation of the kinetics of cell death from heat, radiation, and combined heat‐radiation treatments: direct heat death is rapid and essentially complete within 24‐48 hours after heat exposure, whereas radiation death develops only after a delay period of several days. Radiosensitization by heat affects only the delayed component of cell death, that is, the radiation component of death. (2)Thermal radiosensitization and direct heat death as a function of heating time: thermal radiosensitization requires only short exposures to heat; prolonged heating does not further enhance this effect. In contrast, direct heat death increases with increasing duration of heat exposure. (3)Independent modification of radiosensitization and thermal death: addition of 5% glycerol to the incubation medium protects cells against direct heat death, but not against thermal radiosensitization. In combination, these findings suggest that heat potentiation of radiation lethality and direct heat death are two distinct phenomena mediated by different cellular mechanisms.