Postirradiation Treatment Of Cells Research Articles

Repair of Potentially Lethal Damage in X-Irradiated HeLa Cells1

Although it has been known for several years that mammalian cells cultured in vitro are able to repair sublethal damage from ionizing radiations (1), only a few attempts have been made to examine the capacity of these cells to repair damage which is potentially lethal-that is, damage which ordinarily causes cell death, but whose expression can be prevented by appropriate postirradiation treatment. In contrast, numerous studies have been concerned with modifications of the survival of irradiated microorganisms by various postirradiation treatments, such as incubation at suboptimal temperatures (2-4) or in minimal medium (3, 4), or treatment with inhibitors of protein synthesis (5, 6). These have led to the conclusion that postirradiation repair processes occur in microorganisms. The few studies carried out with cultured mammalian cells suggest that similar repair processes may also occur in these cells. Thus, it has been observed that incubation at low temperature can cause either an increase (7, 8) or a decrease (9, 10) in cell survival, although there may be no effect except a delay in the repair of sublethal damage (11). Similarly, supplementation of the culture medium with a number of complex substances has been reported to enhance cell survival following exposure to ionizing radiation (12-18). Finally, postirradiation treatment of cells with inhibitors of RNA, DNA, or protein synthesis has been found, in some instances, to alter cell survival (8, 19, 20). While studying cyclical fluctuations in radiation sensitivity with the synchronous HeLa S3 system, it was observed that postirradiation treatment of cells with several agents caused significant alteration in cell survival. The results of an investigation

Half-life values for DNA double-strand break rejoining in yeast can vary by more than an order of magnitude depending on the irradiation conditions.

Yeast cells in stationary phase were exposed under oxic or anoxic conditions to sparsely (30 MeV electrons) or densely (3.5 MeV alpha-particles) ionizing radiation. For all four experimental set-ups postirradiation treatment of cells was the same, i.e. cells were kept under oxic conditions in non-growth medium at 30 degrees C. Double-strand break (dsb) rejoining was measured during this treatment yielding the following results: (1) half-life values ranged from < 60 min (electrons, anoxia) to 3.8 h (low doses of electrons, oxia), 7 h (alpha-particles, anoxia), 10 h (high doses of electrons, oxia) and 13 h (alpha-particles, oxia). (2) In the case of exposure of oxic cells to electrons a biphasic rejoining kinetics is observed with a dose-dependent increase of the fraction of the slow component. These results suggest that half-life values of dsb rejoining in a given cell depend on physical, chemical and biological parameters. The rejoining of dsb slows down with increasing LET, being probably due to the increasing complexity of dsb. Oxygenation of cells at the time of irradiation affects half-life values, indicating that radiation chemistry plays an important role. The biphasic rejoining kinetics observed for dsb induced by electrons in oxic cells is interpreted in terms of a dose-dependent change of chromatin structure hindering the interaction between damaged chromatin and the rejoining enzymes rather than by two chemically distinct types of dsb with differing half-life values.

Repair and fixation of potentially lethal damage (PLD) as demonstrated by delayed plating or incubation with araA in contact inhibited refed plateau-phase C3H mouse embryo 10 T1/2 cells grown in the presence of BrdUrd.

C3H mouse 10 T1/2 cells showing strong inhibition of growth at confluency were grown under daily refeeding in the presence of BrdUrd (from 0 to 1 microM) and exposed to gamma-rays either while exponentially growing or in the plateau phase. An increase in radiosensitivity was observed in both growth conditions mainly reflected by a reduction in Dq. Greater radiosensitization was observed in exponentially growing than in plateau-phase cells, and 3-4 times higher BrdUrd concentrations were required in plateau-phase cells for similar potentiation in killing. This effect could not be entirely attributed to a reduction in BrdUrd incorporation since measurements with 3H-BrdUrd showed reductions in incorporation between only 17-47% in plateau-phase cells. The rate of repair of potentially lethal damage (PLD) as demonstrated by delayed plating was not affected by the incorporation of BrdUrd, but the amount of repair (measured as the relative increase in cell survival) was higher for BrdUrd-containing cells. Post-irradiation treatment of cells in the plateau-phase (no BrdUrd) with 9-beta-D-arabinofuranosyladenine (araA) caused fixation of radiation-induced PLD. AraA treatment of cells grown in the presence of various amounts of BrdUrd also caused fixation of PLD, but resulted in survival levels similar to those observed with cells growing in BrdUrd-free medium. This result indicates that BrdUrd mediated radiosensitization cannot be observed when cells are prevented from repairing PLD by postirradiation incubation with araA. Based on these findings we propose that the mechanism of radiosensitization by BrdUrd incorporation might be, by increasing probability of fixation, mediated by the postirradiation progression of cells through the cycle, of a sector of PLD also sensitive to post-irradiation treatment with araA. For this sector of PLD the term alpha-PLD has been proposed.

Chromatid aberrations in a novobiocin-resistant subline of Chinese hamster V79 cells after exposure to novobiocin

We examined effects of novobiocin alone or in combination with γ-irradiation, on the frequencies of chromatid-type aberrations in a novobiocin-resistant subline of Chinese hamster V79 cells (NOVO r-1). NOVO r-1 cells were significantly resistant to novobiocin, as compared to wild-type V79 (WT) cells, with respect to cell survival and DNA synthesis. Survival responses of WT and NOOV r-1 cells to γ-rays in the range 2–10 Gy differed only slightly and the number of chromatid aberrations produced by irradiation at 1 Gy was fairly comparable in the two cell types. Post-irradiation treatment of cells with novobiocin at concentrations exceeding 200 μg/ml significantly increased the number of chromatid gaps plus breaks in WT cells as compared with NOVO r-1 cells. With 200 μg/ml the increase was 1.2-fold ( t-test, P < 0.05) and with 400 μg/ml, 2.3-fold ( P < 0.01) the number produced in NOVO r-1 cells.

Radiosensitizing and Toxic Effects of the 2-Nitroimidazole Ro-07-0582 in Hypoxic Mammalian Cells

The properties of the electron-affinic 2-nitroimidazole Ro-07-0582 have been examined in vitro in two Chinese hamster cell lines, CHO and CH2B/sub 2/. The drug was found to sensitize hypoxic cells to radiation damage selectively with a high efficiency. At concentrations greater than 10 mM the drug has a dose-modifying factor of approximately 3.0 in hypoxic cultures. It does not alter the response of aerobic cells. Pre- or postirradiation treatment of cells with Ro-07-0582 has little, if any, effect. At high cell concentrations, where many sensitizers are ineffective, this compound retains most of its radiosensitizing capability. Ro-07-0582 is also shown to have a chemotherapeutic potential in that it demonstrates a very selective toxicity for hypoxic cells after a few hours of exposure. It is much less toxic to aerobic cells. This property, while it may serve as a useful complement to the radiosensitizing effect of the drug, suggests that care must be exercized in the clinical application of Ro-07-0582 to avoid possible injury to normal hypoxic tissues in the body.

Cellular Effects of Ionizing Radiation

Radiation ModifiersA variety of substances have been shown to alter lethal effects when present in cells before or during irradiation. They can be logically thought of in two classes: compounds offering protection against the effects of radiation; and compounds sensitizing cells to irradiation. Oxygen is perhaps the most important example in the latter group. While considering the major groups of modifiers of current interest, I shall not discuss alterations in survival produced by postirradiation treatment of cells; these effects involve either intracellular repair processes or the metabolic development of the initial damage. Examples of such treatments include postirradiation changes . . .