Antagonization by cycloheximide of ethyl methanesulfonate-induced 6-thioguanine-resistant mutation and sister-chromatid exchanges

Isolation and characterization of Chinese hamster cell line resistant to monofunctional alkylating agents

A clonal derivative of a Chinese hamster Don D-6 cell line resistant to methyl methane sulfonate (MMS) has been isolated following mutagenesis by ethyl methane sulfonate (EMS). The clone, designated as MMSr-1, exhibited high resistance to killing by the monofunctional alkylating agents MMS and EMS. This characteristic had not been acquired by a transient adaptation to the alkylating agents but was found to be a stable heritable trait. MMSr-1 was more sensitive to high-molecular-weight chemicals, such as colchicine and puromycin, than Don D-6. Both MMSr-1 and its parental cells showed the same ability to take up radioactive MMS. The resistance of MMSr-1 appears not to be due to altered uptake of MMS. The resistance was accompanied by low chromosomal aberration and sister chromatid exchange (SCE) induction but not by mutability. Protein synthesis inhibitors such as cycloheximide and puromycin reduced the resistance to the same level as that in Don D-6. SCE induction by MMS in this clone was not antagonized by the protein synthesis inhibitors, whereas mutagenesis was reversed to the normal parental cell level by these inhibitors. Aphidicolin, a DNA-synthesis inhibitor, exhibited no such effects. These results suggest that MMSr-1 might have modified repair capacity, which can be normalized by treatment with the protein-synthesis inhibitors, for lethal DNA damage by monofunctional alkylating agents, and that SCE formation by the alkylating agents is closely correlated with chromosomal aberration and cell lethality.

Sister chromatid exchange in human lymphocytes exposed to ascorbic acid and the cancer chemotherapeutic agent 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea

This study examines sister chromatid exchange (SCE) induction by ascorbate, a weak in vitro SCE inducer which acts through free radical intermediates, and low doses of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), a potent SCE inducer which acts primarily through DNA interstrand cross-links. A small dose-dependent increase in SCE was observed in human peripheral lymphocytes exposed to ascorbate in the 0.5-10 mM dose range for 59 hours, with significant slowing of cell cycle kinetics at concentrations at and above 5 mM. CCNU concentration was selected to approximate the maximal increase in SCE induced by ascorbate. SCE frequencies in cells exposed sequentially to both agents were not significantly different from expectations under an additivity-of-effect model based on SCE response to each agent individually. Despite clear differences in the types of lesions induced, ascorbate and CCNU appear to act independently to induce SCE in a manner consistent with, though not exclusive to, Painter's replicon cluster model.

The influence of a protein synthesis inhibitor on sister-chromatid exchange in the plant Vicia faba

Cycloheximide strongly antagonizes the induction of sister-chromatid exchange by mitomycin C in Vicia faba root tips. This behavior is analogous to that previously observed in mammalian cells (Sono and Sakaguchi, 1981) and suggests that newly synthesized protein is also required for recombination between sister DNA molecules in plants. Conversely hydroxyurea is shown to increase the frequency of both spontaneous and induced sister-chromatid exchange. Based on these results, possible mechanisms underlying sister-chromatid exchange formation in plants are discussed with special emphasis on the absence of DNA polymerase beta in somatic tissues.

Sister-chromatid exchanges and cell-cycle kinetics in human lymphocyte cultures exposed to alkylating mutagens: apparent deformity in dose-response relationships

Experiments have been carried out using human whole-blood cultures to determine the effects of sampling times and of the duration of 5-bromodeoxyuridine (BrdUrd) treatment before fixation on sister-chromatid exchange (SCE) frequencies following exposure to mitomycin C (MMC). Cells were pulse treated for 1 h with 3 X 10(-6) M MMC at G1, and then sampled at 4-h intervals up to 88 h after stimulation of cultures with phytohemagglutinin (PHA). Results showed that this MMC treatment induced a 5-6 h proliferation delay per cell cycle, and that SCE frequencies first increased with time of fixation, peaking at 68 h, and then decreased. When cells were similarly treated with MMC, but subsequently exposed to BrdUrd for various times before fixation of cultures at 72 h, the SCE frequencies markedly increased with increasing durations of BrdUrd incubation times. These data indicate that, in mutagen-treated cultures, lymphocytes having relatively longer cell-cycle times show a higher mean frequency of SCEs. In a subsequent experiment, cells were treated for 1 h with increasing doses of MMC or 4-nitroquinoline 1-oxide (4NQO) at 0, 24, or 48 h, and then fixed at 72 h after PHA stimulation. Results showed that the optimal treatment times at which the agents could most efficiently produce SCEs were different for MMC and 4NQO, and that the dose-response curves tended to 'bend down' at very high doses; that is, treatments with very high doses induced smaller than expected numbers of SCEs. However, cells similarly treated with very high doses showed a higher, expected frequency of SCEs when sampled at 84 h, but again had a lower than expected SCE frequency when fixed at 96 h. The results indicate that there is an optimal time for sampling at which one can observe the maximum increase in SCE frequencies following mutagen exposure, and strongly suggest that the higher the dose, the later the optimal sampling time. Because of the apparent deformity of dose-response curves obtained after various treatments and sampling times, it seems necessary that extra fixation-time points be included in test protocols so as to avoid false negatives or confirm possible positives.

Isolation of Chinese hamster cells hyposensitive to ethyl methanesulfonate and their characterization in induction and antagonization of sister-chromatid exchanges

To better understand the molecular events involved in the formation of SCEs and mutations, 3 clonal sublines were established from a Chinese hamster cell line, Don D-6, on the basis of hyposensitivity to killing by ethyl methanesulfonate. These sublines, referred to as T80-4, T80-6 and T80-7, exhibited greatly altered karyotypes and altered frequencies of SCEs and mutations. These sublines were also less sensitive to chemical mutagens and ultraviolet irradiation as indicated by reduced frequencies of SCEs. This lower sensitivity was not due to differences in the rate of cell proliferation or to a cellular adaptive response. The rate of chemically- or UV-induced SCEs in these sublines did not always relate to cellular ploidy levels. The recent observation that, under constant experimental conditions, the rate of SCEs is a function of cellular ploidy level was not observed for these sublines. Thus, there appeared to be a modification in the intrinsic process of SCE formation. We reported previously that protein synthesis is essential for the induction of mutation as well as for SCE in Don D-6. However, the ability of cycloheximide (CH) to antagonize EMS-induced SCEs and mutations in the T80-7 subline was much lower than in the parental cells or in other sublines. The cells of this subline might contain a defective protein or a modification of the process for which the protein is required.