Effect of membrane lipid perturbers on the temperature dependence of repair of sublethal and potentially lethal radiation damage

Use of the in vivo skin comet assay to evaluate the DNA-damaging potential of chemicals applied to the skin

The aim of the present study was to evaluate both sensitivity and specificity of an in vivo skin comet assay using chemically treated, hairless mouse dorsal skin as a model. N-methyl-N'-nitro-N-nitrosoguanidine (MNNG, 0.0125-0.2%), 4-nitroquinoline-1-oxide (4NQO, 0.01-0.25%), mitomycin C (MMC, 0.0125-0.05%), benzo[a]pyrene (B[a]P, 0.25-2%), and 7,12-dimethylbenz[a]anthracene (DMBA, 0.25-1%) were each applied once to the dorsal skin of hairless male mice; after 3h, epidermal skin cells were isolated, and the alkaline comet assay was performed. The assay was performed after 24h for only the B[a]P and DMBA. Furthermore, B[a]P and DMBA were evaluated by alkaline comet assay using liver cells after both 3 and 24h. The mean percent of DNA (%DNA) in tail in the 0.05-0.2% MNNG and 0.1-0.25% 4NQO treatment groups was markedly higher than in the control group at 3h post-application. Although the mean %DNA values in the tail in the B[a]P and DMBA groups were the same as the controls at 3h post-application, the 2% B[a]P and 1% DMBA groups showed significantly higher values versus controls 24h after application. No significant increases in the mean %DNA in the tail were observed in the MMC group. No clear increases in %DNA in the tail were observed in the B[a]P and DMBA groups at 3 or 24h after application in the liver. These results suggest that the in vivo skin comet assay is able to accurately identify DNA-damaging potential with a skin-specific response and is a useful method to detect the DNA-damaging potential of genotoxic chemicals on the skin.

Flow cytometric analysis of the cell cycle phase specificity of DNA damage induced by radiation, hydrogen peroxide and doxorubicin

We have optimized a flow cytometric DNA alkaline unwinding assay to increase the sensitivity in detecting low levels of DNA damage (strand breaks and alkali-labile sites) and to permit the measurement of the extent of DNA damage within each cell cycle compartment. The lowest gamma radiation dose that induced detectable DNA damage in each cell cycle phase of HeLa and CEM cells was 10 cGy. The lowest H(2)O(2) concentration that induced detectable DNA damage in each cell cycle phase was 0.5 microM in HeLa cells, and 1-2.5 TmicroM in CEM cells. For both HeLa cells and CEM cells, DNA damage in each cell cycle compartment increased approximately linearly with increasing doses of gamma radiation and H(2)O(2). Although untreated HeLa and CEM cells in S phase consistently exhibited greater DNA unwinding than did G(1) or G(2) cells (presumably due to DNA strand breaks associated with replication forks), there was no difference between the susceptibility of G(0)/G(1), S and G(2)/M phase cells to DNA damage induced by gamma radiation or H(2)O(2), or in the rate of repair of this damage. In each cell cycle phase, the susceptibility to gamma radiation-induced DNA damage was greater in CEM cells than in HeLa cells. In contrast to the lack of cell cycle phase-specific DNA damage induced by exposure to gamma radiation or H(2)O(2), the cancer chemotherapeutic drug doxorubicin (adriamycin) predominantly induced DNA damage in G(2) phase cells.

Comet assay detects cold repair of UV-A damages in a human B-lymphoblast cell line

During DNA repair studies, cells are occasionally kept on ice in order to suppress DNA repair. In the present studies cultivated human NC37 B-lymphoblasts were damaged by UV-A irradiation (365 nm) and DNA single strand breaks were detected at the single cell level with the alkaline comet assay in the temperature range from 4 degrees C to 44 degrees C. Single cell studies, in contrast to bulk experiments, allow to identify apoptotic or necrotic cells, which can be omitted for data analysis. Unexpectedly, similarly efficient single phase repair kinetics was found at all temperatures below 37 degrees C, i.e., particularly also in the cold. For recovery times below 20 min a linear decrease of DNA damage was detected. After 20 min, no additional repair was observed, i.e., complete repair of single strand breaks was not achieved. At 44 degrees C DNA damage increased with time, probably due to heat damage and cell death. Nucleotide excision repair inhibitors such as aphidicolin, 1-beta-D-arabinofuranosyl cytosine (araC) and hydroxyurea, but not the base excision repair inhibitor methoxyamine caused a strong increase in DNA strand breaks. The use of repair inhibitors confirmed DNA repair at 4 degrees C. In conclusion, partial repair of UV-A damage is similar at 37 degrees C and 4 degrees C and is probably governed by nucleotide excision repair. Keeping samples on ice may not result in a total suppression of DNA repair.

Hypothermia-enhanced human tumor cell radiosensitivity

Ablation of neoplastic disease by freezing has found increasing utility as a potential therapeutic modality. To assess the effect of cooling temperatures on cellular radiation response, an established human cervical carcinoma cell line (HTB35) was subjected to holding temperatures of 0, 5, or 15 degrees C for up to 24 h before irradiation. Survival was measured by in vitro clonogenic assay of colonies containing at least 50 cells. Cooling for up to 12 h did not significantly decrease survival, but after 24 h survival fell to 75% of control cultures grown at 37 degrees C. X-irradiation immediately after cooling for 24 h resulted in 1.6-fold enhanced radiosensitivity. However, the radiosensitizing effect decayed rapidly if the cooled cells were returned to normal growth temperature for 6 h or longer before irradiation and subculture. Both temperature and cooling duration influenced the radiation response. With 0, 5, or 15 degrees C, radiosensitivity increased after 3, 6, or 12 h, respectively, and progressively rose with up to 24 h of cooling. By flow cytometric analysis, no statistically significant difference was observed in the S-phase fraction between control cells and those cooled to 0 degree C for 24 h. These data demonstrate cooling-enhanced in vitro radiation sensitivity which is dependent upon cooling temperature, duration, and rewarming interval before irradiation. While cell cycle redistribution does not appear to be a factor in the increased radiosensitivity, differences in the radiation survival curves between cooled versus normothermic cells suggest that diminished capacity for sublethal damage repair may be a significant influence on the changes which were observed.

Effects of butylated hydroxytoluene on membrane lipid fluidity and freeze-thaw survival in mammalian cells

Butylated hydroxytoluene (BHT) increases the fluidity of membrane lipids in the hydrocarbon but not the polar regions, as measured by electron spin resonance spin label probes. BHT also sensitizes nucleated mammalian cells to freeze-thaw damage as measured by colony formation survival assays. Furthermore, the membranes of BHT-exposed cells are more susceptible to physical stress, as reflected by the BHT-induced sensitization to hypotonic stress. Since others have shown that BHT induces hexagonal phase lipids in lipid bilayers, this phenomenon may also influence the above survival results.

Effects of pre- and post-thaw cell-to-cell contact and trypsin on survival of freeze-thaw damaged mammalian cells

When multicellular spheroids, which simulate small bits of tissue, are exposed to a freeze-thaw (FT) cycle, the survival of the individual cells in the spheroid is higher if the cells of the spheroid are trypsinized and plated as single cells immediately after thawing than if the spheroid is allowed to remain intact for 4 hr and then trypsinized for plating. The results imply either that cell-to-cell contact inhibits repair of potentially lethal damage (PLD) or that accumulation of additional lethal or sublethal damage during the post-thaw period for cells in contact is taking place. Pre- and post-FT trypsinization of single cells indicate that trypsin does not enhance repair of PLD caused by a FT cycle.

Differential polarized phase fluorometric studies of the perturbation of phospholipid packing by BHT

Butylated hydroxytoluene (BHT) partitions effectively into lipids and modifies cellular function and behavior in numerous ways, particularly at low temperatures. In this study, the modification by BHT of the lifetime and dynamic rotational characteristics of diphenylhexatriene (DPH) in dipalmitoylphosphatidylcholine (DPPC) liposomes was investigated by phase fluorometry. BHT broadens the gel to liquid crystalline transition of DPPC by decreasing the lower end of the transition (T1), leaving the upper end (Th) unaffected. The lifetime of DPH in DPPC does not vary linearly with temperature but declines sharply above the main transition. The average lifetime of DPH is decreased to a greater extent below, compared to above, Th by BHT. This may be due to increased water penetration into the bilayer interior. Non-monoexponential decay also occurs below Th, probably due to a heterogeneous distribution of BHT, which would result in highly perturbed regions of greater than average BHT content. The motional parameter of DPH most affected by BHT is the order parameter (S) which decreased to a considerable extent below Th but is not affected above Th. In contrast, the rotational diffusion coefficient (R) of DPH is increased slightly above Th by BHT.