Mammalian cells: damage in late replicating DNA as the most efficient cause of reproductive death

Toxicity and mutagenicity of X-rays and [125I]dUrd or [3H]TdR incorporated in the DNA of human lymphoblast cells

We measured the toxicity and mutagenicity induced in human diploid lymphoblasts by various radiation doses of X-rays and two internal emitters, [125I]iododeoxyuridine ([125I]dUrd) and [3H]thymidine ([3H]TdR), incorporated into cellular DNA. [125I]dUrd was more effective than [3H]TdR at killing cells and producing mutations to 6-thioguanine resistance (6TGR). No ouabain-resistant mutants were induced by any of these agents. Expressing dose as total disintegrations per cell (dpc), the D0 for cell killing for [125I]dUrd was 28 dpc and for [3H]TdR was 385 dpc. The D0 for X-rays was 48 rad at 37 degrees C. The slopes of the mutation curves were approximately 75 x 10(-8) 6TGR mutants per cell per disintegration for [125I]dUrd and 2 x 10(-8) for [3H]TdR. X-Rays induced 8 x 10(-8) 6TGR mutants per cell per rad. Normalizing for survival, [125I]dUrd remained much more mutagenic at low doses (high survival levels) than the other two agents. Treatment of the cells at either 37 degrees C or while frozen at -70 degrees C yielded no difference in cytotoxicity or mutation for [125]dUrd or [3H]TdR, whereas X-rays were 6 times less effective in killing cells at -70 degrees C. Assuming that incorporation was random throughout the genome, the mutagenic efficiencies of the radionuclides could be calculated by dividing the mutation rate by the level of incorporation. If the effective target size of the 6TGR locus is 1000-3000 base pairs, then the mutagenic efficiency of [125I]dUrd is 1.0-3.0 and of [3H]TdR is 0.02-0.06 total genomic mutations per cell per disintegration. 125I disintegrations are known to produce localized DNA double-strand breaks. If these breaks are potentially lethal lesions, they must be repaired, since the mean lethal dose (D0) was 28 dpc. The observations that a single dpc has a high probability of producing a mutation (mutagenic efficiency 1.0-3.0) would suggest, however, that this repair is extremely error-prone. If the breaks need not be repaired to permit survival, then lethal lesions are a subset of or are completely different from mutagenic lesions.

Cell cycle phase specificity of hydroxyurea and its effects on the cell kinetics in embryonic spinal cord

Pregnant mice (NMRI) received an intravenous injection of 500 mg/kg of hydroxyurea (HU) on day 10 of gestation. The animals were killed 1, 1.5, 2, 2.5, 4, 6, 8 or 10 h after application of the drug and the embryos removed. Forty-five min. prior to sacrifice, the animals were given an intraperitoneal injection of 5 microCi/g 3H-thymidine (specific activity 5 Ci/mmol). From autoradiographs of the embryos the percentage of mitoses, necroses and labelled cells of the spinal cord was determined at the various time intervals. It was observed that HU inhibited DNA-synthesis in all S-phase cells for more than 4 hours. All cells damaged by HU did not start DNA-synthesis again and became necrotic. Only cells entering the S-phase after the influence of HU could replicate their DNA in a normal manner. It is assumed that HU possibly blocks cells in the G2-phase and in the mitosis for a certain period of time.

"Physiological" and cytotoxic cell death in protein deficiency. A study in developing cerebellum in rats

Effects of undernutrition on natural ("physiological") and cytotoxic cell death due to X-irradiation was studied in developing cerebellum in rats. Undernutrition was induced right from birth by doubling the litter size. Animals reared in a litter of 8 served as controls. In order to get an idea of the relationship of cell death to phases of the cell cycle, the number of labelled dead cells, in the EGL of the fissure prima, was estimated, at time intervals following administration of 3HTdR in a group of 9-day-old rats. No differences were observed in the quantum of "physiological" cell death as well as in the rate of nuclear autolysis in the two groups. Undernutrition, however, prolonged the G2-M junction in animals X-irradiated with 200 rads. The kinetics of appearance of labelled dead cells suggested that irrespective of the diet the cells die immediately following mitosis, although the "decision to die" was taken at the end of S-phase. It is concluded that the reduction in cell number in the brain in undernutrition is not due to increased cell death.

Investigation of the effect of hydroxyurea on the cell cycle and the development of necrosis in the embryonic CNS of mice

On day 10 of gestation pregnant mice (strain NMRI) were given an intravenous injection of 500 mg/kg Hydroxyurea (HU). Simultaneously either 5 muCi/g or 10 muCi/g 3H-thymidine (3H-Tdr; specific activity 5Ci/mmol) was administered to the animals. At various times after treatment embryos and electron microscopy. Two hours after administration of HU condensations of the chromatin structure could be detected electron microscopically in some cells. Thirty minutes later the nucleolus became smaller and denser, the cytoplasm shrank, and the cell organelles moved closer together. Three hours after application of the drug break-down of the involved cells set in. As in the autoradiograms about 98% of the counted necroses were labelled, and since labelled thymidine is almost exclusively incorporated during the S-phase, it can be stated the HU influences only metabolic processes which take place during the S-phase. From the morphological findings it can be concluded that in the case of the S-phase-specific metabolic pathway, which is influenced by HU, we are primarily dealing with DNA synthesis.

The effect of hyperthermia on the neuroepithelium of the 21-day guinea-pig foetus: histologic and ultrastructural study

Hyperthermia was induced in guinea-pigs on day 21 of gestation by placing them in an incubator set at 42-5 degrees-43-0 degrees C for 1 hr. At intervals thereafter foetuses were removed from the uterus and sections of the telencephalon were prepared for light and electron microscopy. The histologic and ultrastructural appearance of the telencephalon of the normal 21-day guinea-pig foetus was described for comparative purposes. Damage to cells in mitosis characterised by clumping of chromosomes, and dispersal of polysomes in interphase cells were observed immediately after hyperthermia. Breakdown of the network of junctional complexes was apparent at 4 hr and cellular proliferation was inhibited for 6-8 hr. Degenerative changes and cell deaths were observed deep in the venticular zone after 8 hr; the extent of cell death was related to the post-stressing temperature. Proliferation was resumed at 8 hr and damaged and dead cells moved outward toward the intermediate zone. Phagocytosis of debris by large mononuclear cells was a common finding. Cytoplasmic inclusions, some of which were Feulgen-positive, were present in otherwise normal ventricular cells. Occasional dead cells and empty spaces were present in the ventricular zone at 24 hr and by 48 hr the ventricular zone was normal in appearance. It was concluded that previously observed micrencephaly in the offspring of guine-pig mothers which were heat stressed on day 21 of gestation resulted from a temporary cessation of proliferation and partial depopulation of the proliferating neuroepithelium.

The effect of combined treatment with a platinum complex and ionizing radiation on chinese hamster ovary cells in vitro

Cis-dichlorobis(cyclopentylamine)platinum II (DBCP) belongs to the group of platinum complexes which have recently been reported to have anti-tumour activity. Its cytotoxic activity in CHO cells is not cell-age-dependent, but enhancement of the effect of ionizing radiation is both dose-and cell cycle phase-dependent. In asynchronous cell populations DBCP acts as a dose-modifying factor for ionizing radiation. Doses of DBCP reducing survival of CHO cells to 26% and 4% applied 1 h before irradiation reduce the Do value of radiation dose-survival curves by factors of 1-34 and 1-59 respectively. In synchronized CHO populations enhancement by DBCP of the effect of radiation is most pronounced in G1 and in late S while it is reduced in mid-S. Possible mechanisms of DBCP-radiation interaction are discussed.

The induction by X-rays of chromosome aberrations in male Guinea-pigs, rabbits and golden hamsters. III. Dose-response relationship after single doses of X-rays to spermatogonia

The induction by X-rays of translocations in spermatogonia was studied by cytological means in spermatocytes derived from them. In the rabbit and guinea-pig hump shaped dose-response curves were obtained, with a linear relationship at the low doses. The shapes of the curves were similar to those reported for the mouse, except that the maximum occurred at 600-700 rad in the mouse as opposed to 300 rad in the guinea-pig and rabbit. Unlike the guinea-pig and rabbit, the golden hamster showed a hump dose-response curve without a definite peak value and with little decrease in yield at high radiation doses. Over the low dose range 100-300 rad, the slopes of the curves of translocation yield were in the order:mouse (highest), rabbit, guinea-pig and hamster. Data on sterile periods suggested that the amount of spermatogonial killing in the rabbit and guinea-pig was as great or greater than in the mouse, and that in the golden hamster it was most severe. It is suggested that the differing shapes of the dose-response curves can be explained by a lower sensitivity to translocation induction in the test species and, also especially in the golden hamster, a greater sensitivity to cell killing. The possibility of extrapolating from these data to other species is discussed.