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K G Hofer

Publications and source records attributed to K G Hofer.

At least 37 records · Page 2Linked to original sources
PubMed1988

Cell progression after selective irradiation of DNA during the cell cycle.

Chinese hamster ovary cells were labeled with [125I]iododeoxyuridine (125IUdR, 0.1184 MBq/ml for 20 min) and the labeled mitotic cells were collected by selective detachment ("mitotic shake off"). The cells were pooled, plated into replicate flasks, and allowed to progress through the cell cycle. At several times after plating, corresponding to G1, S, late S, and G2 plus M, cells were cooled to stop cell cycle progression and to facilitate accumulation of 125I decays. Evaluation of cell progression into the subsequent mitosis indicated that accumulation of additional 125I decays during G1 or S phase was eight to nine times less effective in inducing progression delay than decays accumulated during G2. The results support our previous hypothesis that DNA damage per se is not responsible for radiation-induced progression delay. Instead, 125I-labeled DNA appears to act as a source of radiation that associates during the G2 phase of the cell cycle with another radiosensitive structure in the cell nucleus, and damage to the latter structure by overlap irradiation is responsible for progression delay (M. H. Schneiderman and K. G. Hofer, Radiat. Res. 84, 462-476 (1980].

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PubMed1987

Heat potentiation of radiation damage versus radiation potentiation of heat damage.

The enhanced lethality of mammalian cells after combined treatment with hyperthermia and radiation is usually attributed to heat potentiation of radiation damage. However, it has been suggested that the situation may be reversed and that radiation may act as a modifier for heat damage. To test this hypothesis, BP-8 murine sarcoma cells were subjected to sequential radiation and heat treatments and the kinetics and extent of cell death were evaluated with the [125I]-iododeoxyuridine prelabeling assay. Cell death after heating was rapid and essentially complete within 2 days after heat exposure, whereas radiation death was slow and became apparent only after a delay period of 3 days. Combined exposure of cells to radiation and heat caused a pronounced increase in the delayed component of cell death, that is, the radiation component of death. Irradiation of cells before heating did not change the early heat component of cell death even in cells that were exposed to massive radiation doses of up to 300 Gy prior to heating. These results indicate that the increased cell death observed in hyperthermia/radiation-treated cells results from heat potentiation of radiation damage, not radiation potentiation of heat damage.

Animals↗
PubMed1987

Radiosensitizing and cytocidal effects of misonidazole: evidence for separate modes of action.

Hypoxic BP-8 murine sarcoma cells were exposed to misonidazole and/or radiation and the kinetics and extent of cell death were evaluated with the [125I]iododeoxyuridine-prelabeling assay. Cell death after treatment with lethal doses of misonidazole was rapid and essentially complete within 2 or 3 days after drug exposure. In contrast, radiation death became apparent only after a delay period of 4 days and was complete by Day 10 after irradiation. Radiosensitization by short exposures to sublethal doses of misonidazole affected only the delayed component of cell death, that is, the radiation component of death. In experiments involving sequential radiation and drug treatment, prior irradiation of cells did not enhance the direct cytocidal effects of misonidazole, as evidenced by the fact that the early component of cell death was equal in control and preirradiated cells. However, postirradiation treatment with misonidazole did enhance the delayed radiation component of cell death. These results suggest that radiosensitization and direct killing by misonidazole are two distinct phenomena mediated by different cellular mechanisms, and radiosensitization by misonidazole represents a two-component effect composed of true dose modification and dose additive damage interactions, but these additive effects must occur at a site different from the cellular structure responsible for direct drug-induced cell death.

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PubMed1986

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.

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PubMed1985

Cell lethality after selective irradiation of the DNA replication fork.

It has been suggested that nascent DNA located at the DNA replication fork may exhibit enhanced sensitivity to radiation damage. To evaluate this hypothesis, Chinese hamster ovary cells (CHO) were labeled with 125I-iododeoxyuridine (125IUdR) either in the presence or absence of aphidicolin. Aphidicolin (5 micrograms/ml) reduced cellular 125IUdR incorporation to 3-5% of the control value. The residual 125I incorporation appeared to be restricted to low molecular weight (sub-replicon sized) fragments of DNA which were more sensitive to micrococcal nuclease attack and less sensitive to high salt DNase I digestion than randomly labeled DNA. These findings suggest that DNA replicated in the presence of aphidicolin remains localized at the replication fork adjacent to the nuclear matrix. Based on these observations an attempt was made to compare the lethal consequences of 125I decays at the replication fork to that of 125I decays randomly distributed over the entire genome. Regardless of the distribution of decay events, all treatment groups exhibited identical dose-response curves (D0: 101 125I decays/cell). Since differential irradiation of the replication complex did not result in enhanced cell lethality, it can be concluded that neither the nascent DNA nor the protein components (replicative enzymes, nuclear protein matrix) associated with the DNA replication site constitute key radiosensitive targets within the cellular genome.

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PubMed1985

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.

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PubMed1983

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.

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PubMed1981

Influence of hypoxia and acidity on thermal radiosensitization and direct heat-induced death of BP-8 sarcoma cells.

The response to heat of BP-8 murine sarcoma cells was evaluated with the 125I-iodo-deoxyuridine prelabeling technique. Euoxic and hypoxic BP-8 cells were heated in vitro at pH 7.4 vs. 6.5 at temperatures ranging from 37 degrees C to 44 degrees C. The results indicated that acute hypoxia had no effect on either heat potentiation of radiation damage or direct heat-induced death. However, lowering of media pH increased direct thermal cell death and, to a lesser extent, heat potentiation of radiation lethality. These findings suggest that the enhanced radiosensitization and heat-induced death observed in chronically hypoxic tumor populations may, at least in part, be caused by hypoxia-induced tumor acidification.

Animals↗
PubMed1981

Radiosensitization of tumors and normal tissues by combined treatment with misonidazole and heat.

Combination treatment of mice with misonidazole (0.5 mg/g body wt.) and hyperthermia (41.4 degrees C for 45 mins.) produced dramatic radiosensitization in hypoxic BP-8 murine sarcoma cells. The dose modifying factory (DMF: 4.3) was such that hypoxic BP-8 cells subjected to combination therapy became more radiosensitive than untreated, fully oxygenated cell populations. In contrast, radiosensitization by combination treatment was comparatively minor or completely absent in normal body tissues such as skin (DMF: 1.57), intestine (DMF: 1.0), and bone marrow (DMF: 1.0). These results suggest that simultaneous administration of misonidazole and hyperthermia may prove an effective adjuvant to conventional clinical radiation therapy.

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PubMed1980

Radiation biology and potential therapeutic applications of radionuclides.

This report was prepared at the request of the International Atomic Energy Agency and presented at an Advisory Group Meeting on "Applications of Recent Radiobiological Research in Radiotherapy" held in Vienna, Austria, from 24-28 September 1979. Based on the recommendations of the Advisory Group, the I.A.E.A. has decided to initiate a coordinated international research programme entitled "Exploration of the Possibility of high LET RAdiation for Nonconventional Radiotherapy in Cancers". This programme will explore possible applications of corpuscular radiations (neutrons, protons, accelerated heavy ions, negative pi-mesons, thermal and epithermal neutrons) in cancer therapy. In addition, the programme will include research on in situ radiotherapy with unsealed radionuclides and thermal neutron capture. For further information on the programme, please contact Dr T. IWASAKI, Section of Radiation Biology, Division of Life Sciences, International Atomic Energy Agency, Wagramerstrasse 5, P. O. Box 100, A-1400 Vienna, Austria.

Animals↗
PubMed1978

Radionuclide toxicity in cultured mammalian cells: elucidation of the primary site of radiation damage.

Synchronized suspension cultures of Chinese hamster ovary cells (CHO) were labeled with various doses of 3H-thymidine or 125I-iododeoxyuridine to evaluate the cytocidal effects of intranuclear radionuclide decay. Damage produced by radionuclide decay outside the cell nucleus was studied on cells exposed to 125I labeled, monovalent concanavalin A. After labeling, the cells were resynchronized in G1-phase and incubated for 36 h at 4 degrees C to permit dose accumulation. Cell lethality was evaluated by the standard colony assay. Based on radionuclide incorporation data, cellular dimensions, and subcellular radionuclide distributions, the cumulative dose to whole cells, cell nuclei, and cellular cytoplasm was calculated from the known decay properties of 3H and 125I. As expected, DNA associated 125I (LD50: 60 decays/cell; 45 rad) was much more toxic to CHO cells than 3H (LD50: 1350 decays/cell; 380 rad) 380 rad) or external X-irradiation (LD50: 330 rad). In contrast, membrane associated 125I was surprisingly non-toxic (LD50: 19 600 decays/cell). At 19 600 decays/cell the dose to the cell membrane was approximately 52 krad and the overlap dose into the cytoplasm was about 2470 rad. Even at these high dose levels, membrane damage or cytoplasmic damage apparently did not contribute significantly to radiation induced cell death. With 19 600 decays on the plasma membrane the CHO nuclei received an overlap dose of about 410 rad. As can be seen from the LD50 data for 3H and X-rays, a nuclear dose of 410 rad should be sufficient to account for 50% cell death. These findings indicate that, although intranuclear decay by electron capture is extremely destructive, identical decay events in the plasma membrane cause only minimal cell damage. This parallels our earlier studies on 67Ga labeled leukemia cells which showed that electron capture decay in the cytoplasm is also highly ineffective in killing mammalian cells. It therefore appears that radiation-induced cell lethality in dividing mammalian cells results primarily from nuclear damage. Cytoplasmic or membrane contributions to radiation-induced cell death, if any, must be minimal. By implication, these findings refute the enzyme release hypothesis and similar theories designed to explain mitotic death in terms of cytoplasmic or membrane damage rather than nuclear damage.

Animals↗
PubMed1978

Biological toxicity of Auger emitters: molecular fragmentation versus electron irradiation.

Two hypotheses have been advanced to explain the extreme biological toxicity of DNA associated 125I: (a) high local concentrations of radiation energy from low energy Auger electron; (b) charge-induced molecular fragmentations in the DNA. To distinguish between these two hypotheses an attempt was made to evaluate the molecular events associated with electron capture decay, to calculate the microdistribution of radiation energy after 125I decay, and to relate the microdosimetry data to the biological toxicity of 125I and 3H. The cellular damage produced by the two radionuclides was evaluated on synchronized Chinese hamster ovary cells (CHO) labeled with various doses of 3H-thymidine or 125I-iododeoxyuridine. As expected, 125I (LD50: 45 rad; D0: 74 rad) proved much more toxic to CHO cells than either 3H (LD50: 380 rad; D0: 250 rad) or external X-irradiation (LD50: 330 rad; D0: 230 rad). To evaluate the molecular mechanism of 125I toxicity, iododeoxyuridine labeled with both 125I and 14C was synthesized and the effect of 125I decay on the molecular structure of iododeoxyuridine was studied by monitoring the fate of 14C activity after 125I decay. The results of this experiment indicate that 125I decay does not cause molecular fragmentation in iododeoxyuridine, only deiodination. Moreover, microdosimetry calculations show that at least in small target spheres more radiation energy is deposited on the average by decaying 125I than by a high LET alpha-particle traversing a sphere of equal diameter. These findings greatly strengthen the hypothesis that the high LET-type damage produced by Auger emitters results from high local concentrations of radiation energy rather than from charge-induced fragmentation of the DNA.

Animals↗
PubMed1976

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 degrees 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. LD studies on ICR mice revealed that moderate hyperthermia (41.5 degrees 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 tissues.

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